I'm not signing up for the beta-test team, I just want to make suggestion.
Why don't you release the "Release Candidate" versions before making the actual release?
For example, when you think you're ready to release the next version. You release the "SFML 1.2 RC1". Then you wait. Waiting for a week or two should be enough. If bugs are reported in the RC1, you fix them, and release "SFML 1.2 RC2" and so on. When you reach the stage where no bugs are found in the RC version - you simply rename that RC version to "SFML 1.2" (you simply remove the "RC#" part), and you call that the official stable release.
I think, that would be a lot better, because *anyone* who can download the RC versions would be able to test them, not just some (possibly small) group of beta testers.
Anyway, it's just a suggestion.