Snow Leopard is very smart about how it handles legacy drivers. The kernel can run in either 32- or 64-bit mode, but on most computers it boots into 32-bit mode no matter what (I think the XServe is the only one which defaults to 64-bit). All 64-bit drivers are required to provide 32-bit operation as well, so it's basically always safe to boot 32-bit.
Now, the neat thing is that a 32-bit kernel can still run 64-bit applications! So you can have 32-bit drivers, a 64-bit userspace (the default), and a mixture of 32- and 64-bit applications. Most of the included apps are 64-bit now, and many of Apple's pro apps (Aperture, Logic Pro, and I think Final Cut Pro but I'm not sure) are 64-bit as well. They've also worked it such that the 32-bit kernel can address up to 32GB of RAM rather than the usual 4GB, and it'll be some time before they even make a system that can take more than that.
System Preferences also have a pretty neat fallback - if you need to open a legacy control panel that only works in 32-bit mode, it actually relaunches itself in 32-bit mode.
The only thing that doesn't have a 32-bit fallback or compatibility layer is the screen saver, and that's hardly a vital function.
Most of the remaining differences between 10.5 and 10.6 are cosmetic. So far I haven't run into anything that worked on 10.5 that doesn't work on 10.6. My PreSonus FireStudio and Inspire1394 both work great, as do both of my Wacom tablets. The rare times I bother to use my Contour ShuttlePro2 it works fine, too, and I think that's my only legacy 32-bit driver at this point (and I basically never use the damn thing anyway).