Apple knows best on this. By default, your Snow Leopard system (except for Xserves) boots into a 32-bit kernel, even on the latest Macs. That doesn’t keep you from running 64-bit applications and addressing more than 2 GB of RAM.
The problem is that you may have some kernel extensions that are not 64-bit capable. You can try booting into the 64-bit kernel by holding down the 6 and 4 keys together at boot, but not everything may work correctly. To get a feel for some extensions that would be a problem, take a look at: About This Mac -> More Info -> Software -> Extensions. On my system, several are still flagged as 32-bit only.
System profiler shows whether kernel extensions are 64-bit
For now, you may be wondering how to tell if your system was booted into 32 or 64-bit kernel mode. Just take the same route above, but stop at Software, and look for the “64-bit Kernel and Extensions” flag, circled in red below.
System profiler shows mode booted into
There’s really no stigma attached to running in the 32-bit kernel mode, and, in time, all our kernel extensions will be 64-bit. It’s just one more step in the roadmap to a full 64-bit Mac.