A 32-bit PAE kernel will run 64-bit and has been doing so since Tiger (command line 64-bit). Leopard extended the 64-bit functionality even further. Snow Leopard, from what I have read, extends this even further to the extent of a 64-bit kernel.
A 64-bit kernel must have 64-bit extensions (kexts) from applications to run.
I don't work for Apple and have no inside knowledge of how Apple are approaching the transition to 64-bit so can only offer my own conclusions, but based on Microsoft's offerings of XP-64 and Vista 64, there was a slight issue (understatement of the decade) with driver support on third party hardware and software running under 64-bit, to the extent a lot of people are still using XP and those using Vista (oops. . . they dropped the 'V' word . . .erm . . Windows not XP but newer) are still running 32-bit versions.
I have a test machine running Vis . . . erm. . . Windows current version 64-bit. IE8 64 can not display flash based items properly, as Adobe have not released a 64-bit Flash player plug in for IE8-64 - so you have to run the 32-bit version of IE8 to load flash based content. So those saying everything on Vi. . . the other camps' side is fully 64-bit are mistaken. They are not. Issues still abound - Adobe have had many years to make a 64-bit Flash plug in for XP 64 and Vi . . oh you get the point 64, yet they haven't. Presumably because more people are still running 32-bit variations of the OS and so there is little or no major demand for 64-bit development (yet). This may change with the adoption of Windows 7.
It makes sense to me, to cut down the risks of suddenly unusable peripherals or software to end users, to default Snow Leopard to a 32-bit (PAE) kernel which will allow 64-bit applications (which do not require 64-bit extensions) to run and access the full memory installed on an end users system.
Imagine the furore if Snow Leopard installed by default a 64-bit kernel on all machines and suddenly all your third party apps and devices no longer worked? End users wouldn't be too happy.
Once third party vendors have developed 64-bit drivers and applications, the switch to pure 64-bit kernel operation will be made for systems capable of supporting full 64-bit.
Those that don't support full 64-bit will continue to gain performance improvements through 32-bit PAE.
(Oh it's so easy to get confused as you get older. I hope this makes sense - my head is numb now).
Here's what MacFixIt say about 32 and 64 bit:
"Snow Leopard both 32 and 64-bit?
Unlike previous versions of OS X, in Snow Leopard Apple has finally provided a true 64-bit operating system from the kernel up through the kernel extensions and system processes. However, in reality Snow Leopard is provided in both 32-bit and 64-bit flavors and loads the 32-bit kernel by default, so why would Apple do this after advertising 64-bit capabilities?
The first reason is application compatibility. While 64-bit apps will run on a 32-bit kernel, and 32-bit apps will run on a 64-bit kernel, the "bitness" of the kernel will limit what kernel extensions can be loaded: a 64-bit kernel can only load 64-bit extensions, and a 32-bit kernel can only load 32-bit extensions. Most applications do not have their own kernel extensions they use, but a few rely on them for functionality. For instance, VMWare and Parallels install kernel extensions to provide network environments to the virtual machines, which in their current 32-bit only versions will not load on a 64-bit kernel. As such, if you load a 64-bit kernel you will get an error when trying to run Windows from within the Mac OS.
The second reason for loading a default 32-bit kernel is that some of the first Intel Macs were released with 32-bit Core Duo and Core Solo processors. Even though the requirements for Snow Leopard are an Intel Mac, these early Intel Macs will not run a 64-bit kernel but will run other intel-optimized code. These processors will also only run 32-bit versions of applications so any that do not support 32-bit modes will not run."
Full MacFixIt article here:
http://www.macfixit.com/article.php?story=20090826074252596
I believe Apple know what they are doing and are implementing the 64-bit migration in a way to minimise the possible knock on effects to end users (Remember the 64-bit Windows who can't be named debacle).
I can only guess Apple don't want their end users experiencing the same situation as Microsoft created (and good on Apple for that). Panther was better and faster than Jaguar, Tiger was better and faster than Panther, Leopard was better and faster than Tiger, Snow Leopard will, I am guessing based on previous version improvements, be better and faster than Leopard (regardless of 32-bit or 64-bit kernel use - and will always be better than Windows).
So, to sum up, if you have any Intel Mac, (I'm hoping) Snow Leopard will, in all likelihood, improve the performance your Macs are already achieving, be that on a 32-bit kernel or a 64-bit kernel, 32-EFI or 64-EFI.
Roll on Friday!!!!!!!! What time is my local Apple Store opening?
