Sounds like a very difficult wall to get over. I imagine most people will hear about Flattr through a blog or some service accepting payments with it. Only if they're curious enough about it will they go find out what it is, and then once they understand that they work of registering is so that other people can have their money, they have to still be compelled to signup and use it.

Unless each blog/service using it makes it explicit that they're accepting flattr donations, but nobody wants to be that site sounding like they're begging for handouts.