If you have had any Safari plugins, some of them are only 32bit. They shouldn't load in, but some might cause issues. The most popular is probably Safari Ad Block which will only run in 32bit in Snow Leopard. This means you have to make Safari open as a 32bit application:
AppleMacster,
You are, of course, assuming that the poster's Mac has booted up in 64 bit mode.
I belive you'll find that OS X Leopard Desktop will boot in 32 bit by default unless you tell it otherwise through a two-finger salute on boot. You can verify what mode you've booted up through the System Profiler tool (from memory you click on the "SOFTWARE" header and look at what it says next to "64 bit kernel" ...i.e.YES or NO).