Yesterday Facebook launched HipHop, a sourcecode-converter from php to c++. The set of php functions and constructions is more limited than in standard php.
Are the current popular php frameworks (ZF, CakePHP, symfony) compatible with HipHop? If not, which parts of these frameworks are not usable?
I won't repeat what has already been said, I'll just add that as soon as HipHop is released you can count on the opensource community to submit patches for your favorite framework for increased compatibility with HipHop.
Also, one good way to test whether your favorite framework is compatible or not would be to launch the test suite using HPHPi (you then know where to concentrate your patching effort ;).