Love this concept. Over time they can play with other nudges to get people to sign-up: requiring a Flattr account to skip an interstitial ad, for example (even if you don't ultimately choose to give a share to the content), or even (for some premium/early-access content) requiring a Flattr click.
What's going to be hard is preventing users from devaluing their Flattr clicks to get past Flattr-based paywalls. Simplest version: I have a personal array of strawman sites, which I click on a lot in order to funnel almost all of my Flattr fee back to myself. Then my Flattr clicks are near-worthless to non-me recipients. This can be extended by Flattring friends, Flattr kickbacks, etc.
(1) You and your circle can't get more out than you pay in, or convince other people to give you freely
(2) Who cares about those who would go through so much trouble to save a few bucks? The point is to make it easy for those who want to pay, with subtle encouragements to pay. If the easiest thing is paying, enough people will -- and your network of strawman sites is a fringe rounding error.
This is the direction I thought now-defunct Contenture should have taken. Previous discussion links: http://news.ycombinator.com/item?id=1117012