I stumbled across an article saying that Snow leopard does not run in 64 bit mode by default (except on Xservers).
So I did some digging and this is only partly true. Here are the details of what actually happens but the crux if it is as follows:
When your Mac with Snow Leopard boots up it will boot by default with 32bit version of the Kernel, however all 64 bit applications such as the recompiled Apple apps that come with OS X will still run in 64bit mode. So applications like Finder and Mail will be running the 64bit version.
It is also worth noting that having the kernel running in 32bit mode does not affect Grand Central Dispatch and OpenCL, these will run as they should.
The reason for this is that there is still a large number of Kernel extensions such as hardware & printer drivers that are have not been re-written / re-compiled for 64bit.
[Update 12:40] Here is a link worth reading too. You can “force” boot into 64bit by pressing 6 and 4 when the computer starts up. I will test and confirm tonight.
[Update 21:15] I have tested holding down the ’6′ and ’4′ keys and it does work.