It's only a partial outage (Google says 2% of users - much lower than I suspected).

Mine's been down for about half an hour (I'm in the UK). Someone did an Ask HN where it was mentioned that #gmail was trending on twitter which suggests that the problem is widespread. Edit: it is trending on twitter in the US at least. I didn't bother checking other locations.

I suppose one doesn't need to make the obvious observation that Google are of aware of the problem:

http://www.google.com/appsstatus#hl=en&v=issue&ts=13...

Edit2: It's back up for me. Now we wait to see what caused it, which is where the interesting technical information will emerge.