Not really a surprise...
Given the number of dizzy bird icons I see when I access twitter from a desktop browser (i.e. quite a lot, and I'm not even a heavy twitter user), I'd say they've been exceeding the capacity of their servers for quite some time.
It doesn't even seem to depend on time of day. I've even seen "dizzy bird" at 9am GMT, a time which is usually good for the internet as the Americans are asleep.
Maybe they should try to target the politician loving twitter-bot hoards a little better?