David Empson
Regarding [64-bit kernel support]:
There are some models which have 64-bit EFI but are not able to use the 64-bit kernel. My mid-2007 MacBook Pro (model identifier "MacBookPro3,1") is one such model. If I try to boot into 64-bit kernel, it ends up still being in 32-bit mode. The mid-2007 iMac ("iMac7,1") may fall into the same camp but I haven't confirmed its EFI type.
From other evidence it appears Apple has limited 64-bit kernel support to models introduced in 2008 and later, and so far it is only supported on the iMac, 15"/17" MacBook Pro, Mac Pro and Xserve.
This appears to be tied to the "Penryn" generation of processors, which introduced SSE4 vector instructions. A restriction of this nature allows Apple to make use of SSE4 in the 64-bit kernel.
The precise list of models which can run the 64-bit kernel is on the web page for Markus Winter's Startup Mode Selector, already referenced from this question in the FAQ.