The following is a comment by TheForestFloor to the CNET article 64-bit Snow Leopard defaults to 32-bit kernel:
If you boot into a 64bit kernel, none of your 32bit device drivers will work, so people who are upgrading will struggle. Drivers built for snow leopard will have both 32 & 64 bit code, but until that happens, booting into 64bit by default isn’t the right way to go.
That said, ‘booting into 64bit’ only refers to the kernel. All of the other libraries that apps depend on, AppKit, FoundationKit, libc, etc are all 64 bit, and will load if a 64 bit processor is accessible. Also, any program that was compiled with 64bit code will run in 64bits period, giving you that (2x) improvement.
The biggest problem for users will be that their software (Photoshop, Logic, etc) will be 64 bit, but their plugins might not be. If any program has plugins or extensions that are not 64 bit, it will have to be relaunched into 32bit mode to use them. Contrary to contentcreator’s comments, OSX does not ‘grind a bit’ when loading 64 bit code, nor does it load BOTH 32 and 64 bit. It loads 64bit code if it’s dependent libraries and the processor support it. If not, it degrades gracefully (and speedily) to 32bit code.
That said, nearly all programs 32/64/ppc continue to run well on SL, and all of them get to take advantage of GCD when they run, so they run a bit faster as well.
In the pre-release versions of SL, all of the Macbook Pros, Mac Pros and XServes could be set to boot in 64bit by default (without 32bit device drivers), and the others (with the exception of the MBA and Mini) could be booted with the ‘64′ keys held down. I guess they removed it to keep users from getting confused if their devices stopped working. My guess it will be the default for all machines once the driver community has moved all of their code over.