Berlin is awesome. I know is a question with a very flexible answer but what are the monthly net rates a Py/Django/DB expat developer can expect in an average size company (not boring bigcorp) in 2012?
Very flexible indeed. Anywhere from 30k€/y (entry level, junior dev) till 60-70k€/y (top talent with work experience and some management duties) is possible for a regular full time job - sometimes more, but extreme rates are still pretty uncommon here. (Note though that you've got around the same standard of living here than for double the amount in dollars in the valley.)