I work on Gittip, which is a similar-but-different service. We've been wanting to add Bitcoin support for some time, but due to the recurrent nature of what we do, we've not come up with a good solution. I suspect Flattr is struggling with the issues, and I'm interested to see how they implement it.

If you want to get involved in the conversation around Gittip's Bitcoin implementation, check out Github. There are separate conversations going on for incoming and outgoing payments.