(UPDATE Saturday 4 February 18:45 Shanghai time. The Google.cn service is working normally in China after access was restored late on Friday. The company made a statement early Saturday morning China time (but still early morning California) saying that it was working with the local ISP, presumably China Telecom, to restore service.:

‘Users in China have been experiencing
difficulties accessing Google.cn even in the regions where they were
able to access Google.com. We have been working locally in China to
resolve the issue in conjunction with the local internet service
provider (ISP) where the problems occurred. We believe normal service
has been restored,’ said a Google spokesperson.

‘Our efforts to correct the problems were
made more difficult by the Chinese New Year which meant most offices
were closed. We’d like to apologise for any inconvenience caused.’

It’s still not entirely clear what caused the denial of access for China users. The outage seemed like a classic "Great Firewall" issue. The site was fully accessible early in the day and became inaccessible some time in the afternoon. Yet it remained fully functional outside of Mainland China. That generally indicates a block of some sort has been implemented.
Site blocks are usually adjusted at the start of the month - generally on the first ‘working day.’ Key-word filters are also adjusted at that time, although either may be quickly added in the case of "mass incidents" or other factors. Fons mentioned in an off-blog conversation that as it was still a holiday yesterday, it seems odd that a site block would have been engaged. Though none of us really have a clear idea of when censors return to work after the spring festival. They generally don’t make their holiday schedules public.
Fons is hypothesizing that the block could have been related to a . This seems completely possible and reasonable. Although the sudden nature of the denial of access, its prolonged nature (at least six hours) and the sudden resumption of service still make the outage seem like a classic firewall issue. It had the hallmarks of a targeted event rather than a more general bottleneck issue. Trace-route tests and attempts to load the page throughout the day were consistent in their results. For bottlenecks, the results are much more sporadic.
It is possible that the filters were mis-adjusted in some manner to have caused the site block - this does happen. AsiaPundit has never figured out why the Morgan Stanley Capital International site was  once blocked in Shanghai.
There is a possibility that a block on Google.cn was implemented by request and then, again by request, lifted. The process behind the Great Firewall is mysterious, and the Internet falls under the jurisdiction of several ministries (which often have competing agendas).
A source who spoke with a colleague in London said they ‘thought it was an ISP issue not a technical issue.’ I’m not fully sure what that means, but a traffic-related outage would seem like a very straightforward technical issue, even if it was related to the ISP and not the website’s host. The ISPs also have to filter, what they choose to filter is a political issue rather than a technical one. Further clarification will be sought on Monday.
So, was the censored Google.cn briefly censored by the Great Firewall? The concept of the government blocking the censored Google.cn and continuing to allow access to the uncensored Google boggles the mind. While trying to think why that could happen, AsiaPundit considered that some officials would clearly strongly object to the fact that the portal tells users that results are censored.
Shi Tao, remember, was jailed because he ‘leaked to foreigners’ that the government had ordered local media not to mention the anniversary of the Tienanmen Square massacre. What can be a "state secret" in China can be "common knowledge" elsewhere.
How the Great Firewall works is itself a "state secret." So, if the denial of access to Google.cn was firewall related, AsiaPundit expects that the outage will never be fully explained.
It could have been a deliberate block, an accidental block or as Fons suggested yesterday, it’s possible that whoever is responsible for setting the list of blocked sites just has a sense of humor. It quite  likely was purely technical, but until we have more information we are clueless.
AsiaPundit offers apologies for the vast amount of speculation in this post, but the absense of transparency and disclosure leads to speculation.
Comments are most welcome, particularly from Chinese users who noticed the ‘block’ yesterday or those with technical expertise.

(Disclosure: AsiaPundit does prefer to keep the blog and the day job completely separate. AP is a site that welcomes free expression of the author’s opinion and sensational tabloidism; in the day job both are avoided. The news item that I had linked to above was mostly written by me the author of AsiaPundit, with contributions from London. And off the record (dumbass, you can’t post an off-record comment openly on the internet) Google was pretty open about the problems here and were much more proactive in responding to querries than other similar companies have been when responding to me the media. They were, on Friday, still trying to establish what exactly was happening here.)

(UPDATE 23:30 Shanghai time. Google.cn is up in Shanghai. Service has resumed and the trace route is now showing a rather quick connection to the Mountian View server. No reason for the earlier denial of access has yet been made available, though officials are looking into it.)

Google’s recently launched China portal is currently inaccessible in Shanghai without the use of a proxy server. Using a US-based proxy you get this.:

 

Without the proxy you get this.:

 

A trace route test shows that the path to the site is being lost at the China Net backbone.

 

This is typically what happens when a site is blocked by the Great Firewall. The same trace-route result is achieved on other blocked sites such as http://news.bbc.co.uk. The Beijing-based trace route server isn’t working today but a colleague in Beijing tried to load Google.cn and the result was a "time-out" error, indicating that the site is being blocked.

Ironically, the main page of Google.com can still be accessed.

It’s possible that this is the result of a technical glitch of some sort. Or, what this may mean is that the uncensored Google.com can be accessed while the censored Google.cn is being blocked by the government.

Google has said it is looking into the problem.

AsiaPundit would welcome updates from elsewhere in China.

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