-So SFML 2 is actively maintained on all the platforms, Win/Mac/Linux?
Yes. I'm maintaining the Windows and Linux version, and currently Hiura (he's french) is working on the Mac OS X port.
-How difficult is it porting from SFML 1.6 to SFML2?
I would say it's easy, but it depends what SFML modules and classes you use.
-Sorry for this one =p Any benchmark for when SFML2 will be stable? =D
The performances of the graphics module have already been improved in SFML 2, and it might change again before the public release.
For sure I'll post a benchmark of SFML 1 vs SFML 2 when it's stable