It seems to be really taking off in Germany, probably after a talk at the re:publica conference in Berlin by Peter Sunde [1]

Nearly all German blogs I’m reading now have a flattr button (and nearly everything that can be flattred is in German which could be a problem for international adaption. Or not.)

[1] http://re-publica.de/10/event-list/flattr-social-micro-donat...