Maybe one more thing: There should be a warning when you try to send a thank you message for a month when you already have sent one. Or just disable the button after the message was sent.

The timing of this release is great, because January 11th is “International Thank You Day” (according to some people, though I cannot find anything about it on Wikipedia). Maybe the next Pay a Blogger Day can also be combined with this “Thank You Day”, if it does exist (though I guess it will come into existence if many people will it so).

I don’t really see the point of this feature. In my opinion a Flattr is me thanking them for posting something that I found valuable or interesting, I don’t see the need to thank a thank.
Although, now I do feel tempted to thank the thanks.