I have the same problem :? with an ati card (radeon xpress 1100), 2.6.28 kernel, and SFML2 r1205 (I haven't tried any other revision).
X Error of failed request: GLXBadContext
Major opcode of failed request: 154 (GLX)
Minor opcode of failed request: 3 (X_GLXCreateContext)
Serial number of failed request: 30
Current serial number in output stream: 33
Everything works fine with SFML 1.x.
Edit : the problem is the same with compiz on or off.