Yesterday Zend released the Snow Leopard compatible 64 bit ZendDebugger binary. So I can debug again.
imac:extensions thorsten$ file ZendDebugger.so ZendDebugger.so: Mach-O universal binary with 2 architectures ZendDebugger.so (for architecture i386): Mach-O bundle i386 ZendDebugger.so (for architecture x86_64): Mach-O 64-bit bundle x86_64