I on occasion will boot into back 32 bit for a client project that used SampleTank and older VST plugs etc.
but looks like as of 10.8 (Mountain Lion) that will no longer be possible.
Time to move on.
Read the entire article here:
To test this on any system, reboot with the 6 and 4 keys held down, which will force the system (if possible) to boot into 64-bit mode. Once logged in, open the System Profiler (or System Information) tool and select the Software section. In this section check the status of the 64-bit Kernel and Extensions listing, and if it says “No,” then despite attempting to force the system into 64-bit mode, the system is not capable of doing so.