There may be more recent Apple info on the topic, but the KB article
Mac OS X v10.6: Macs that use the 64-bit kernel (linked to in tacit's first reply above) lists which Macs
support the 64-bit kernel and which Macs
use it by default. It also tells you where to find your Mac's current kernel status (System Profiler>System Software Overview). The article tacit linked to shows how to change your kernel from 32-bit to 64-bit or back.
