There's a little bit of confusion out there on the Web about what parts of Snow Leopard are running in 64-bit mode and what parts are running in 32-bit mode. A report by Thom Holwerda of OS News says that under Snow Leopard, most Macs will boot using a 32-bit kernel and drivers, not a 64-bit kernel and drivers. And Holwerda points out that many Mac models don't have 64-bit EFI, either.
These statements, based on a pre-release copy of Snow Leopard, seem accurate to me. But the implication that the story leaves readers with--that you can't "go 64 bit" or "boot into the 64-bit version of Snow Leopard" if your Mac isn't booting into a 64-bit kernel--is completely wrong.
When Apple talks about Snow Leopard being thoroughly 64-bit savvy, what the company means is that almost every application included in Snow Leopard has been recompiled to run in 64-bit mode. There are two reasons this is a good thing. The first is simple: 64-bit computing is necessary if you want one of the programs on your computer to have access to more than 4GB of RAM. Second, there are some speed boosts associated with running in 64-bit mode. The Intel processors that power Macs have built-in math routines that operate more efficiently in 64-bit mode, processing tasks in fewer steps. That means that certain math-intensive tasks will see a speed boost under Snow Leopard's 64-bit applications.
If you're running a Mac powered by an Intel Core 2 Duo processor or an Intel Xeon processor, your Mac is 64-bit capable. And Snow Leopard runs 64-bit applications regardless of whether it's booting into a 64-bit or 32-bit kernel. In fact, the only big advantage of booting into a 64-bit kernel would be the ability to use more than 32 gigabytes of RAM. There aren't any Macs that can do that now, anyway, due to hardware limitations.
Applications running in Snow Leopard will have access to a full 16 exabyte virtual address space, just the same as if they were running in a 64-bit kernel. As a result, there's very little difference between booting into the 64-bit kernel and the 32-bit kernel in current Mac systems. (This is not to say that there won't be a bigger difference in the future, as RAM sizes continue to grow. But presumably new high-end Mac systems will boot into the 64-bit kernel when the need arises.)
So, bottom line: If you've got a Core 2 Duo or Xeon based Mac -- any Intel Mac not running a Core Duo or Core Solo processor -- you'll be able to run applications in 64-bit mode, which will in turn be able to take advantage of faster 64-bit registers and math routines as well as access massive amounts of memory.
Now all we have to do is wait for Snow Leopard to arrive so we can try out those 64-bit applications for ourselves.