Clint Hocking wrote a blog post about the number of developers in Montreal, and several people have commented with info about other geographical areas.
My guess is there’s between 100 and 200 professional game developers in Vienna, if not all of Austria – I only know of a handful of companies outside of Vienna.
In Germany, I expect the biggest areas to be Frankfurt, Berlin and the Ruhrgebiet. Maybe Hannover, Hamburg (which has a surprising amount of publisher offices though) and Munich. But I have not kept a close eye on the German industry for a long time, so I may be off.