I have an early 2008 (White) Macbook that I did a clean install of Lion on. One thing I noticed when looking around was in the System Profiler, if I click on software it says "64-bit kernel extensions: No". Now I thought Lion only ran in 64bit, and when I boot up, it tells me my Apple USB modem can't be used. I was under the impression that was because the kernel driver for the modem was 32bit only - but System profiler says that 64-bit extensions aren't enabled... I'm a little confused.
I was trying to help someone else figure this out a while ago.
There seems to be a physical restriction on those MacBooks from booting into the 64bit kernel, despite the fact the hardware and EFI support it. Holding '6' and '4' doesn't force it, even adding the kernel flag 'arch=x86_64' to com.apple.Boot.plist doesn't work, the machine just won't boot into 64bit mode.
The iMac 5,1 and the MacPro1,1 only have EFI32, but they're also entirely capable of running 64bit mode, but also can't, though there are other machines that have EFI32 but have 64bit capable hardware that boot into the 64bit kernel mode by default.
Personally, my MacBookPro 4,1 boots into 64bit mode by default.
There is a 32bit kernel mode in Lion, but all the core apps (like finder etc) are only written in 64bit, so you still can't install it on a 32bit machine. Running Lion on a 32bit machine by removing 'platformsupport.plist' only worked in DP1 because Finder still had a 32bit version, but since DP2, this was removed. You can however install Lion on something like an older Core Duo Mac Mini which has been upgraded to a C2D (check out iFixIt, they do really helpful guides on how to achine this, as well as many other things).
Hope this helps mate.