After looking at the Official Twitter Blog, Twitter has announced that they have detected an errant API usage through their Jabber resources.

We found an errant API project eating way too much of our Jabber (a flavor of instant messenger) resources. This activity (which we’ve corrected) had an affect of overloading our main database, resulting in the error pages and slowness most people are now encountering.
We’re bringing services back online now. Some will be slower than others for a while, and we’ll be watching IM and IM-based API clients very closely. We’ll also be taking steps to avoid this behavior in the future.

Hope this was causing their issues earlier this week and they can fix them.