Hi! Just browsing through some English-German forums (I'll be doing undergrad thesis research in Germany [mostly Berlin] and Austria from May-July) and had to reply when I came across your post!
If you haven't found anything yet, PM or email me, and I can give you the contact info for the lady who owns and rents/sublets individual apartments in a building in Prenzlauerberg, near Senefelder Platz (or, two stops from Alexander Platz, as it is continuously described to me

). They are very small apartments (I'm staying in a 1-bedroom), but fully furnished and quite nice! Mine has a kitchen, desk, laptop connections - all I really need!

As I mentioned, I'll be renting one for about nine weeks (from a fellow American who sublets the apartment when she's in the U.S.), but I know there are many others available (well, I don't know specific DATES of availability, of course!, but they exist) for short or extended stays, because a friend of mine is also renting one for two weeks in July.
Anyway. Just throwing that out there, in case you're still looking!