Not only are there long lists of hardware that doesn't have 64-bit drivers, but there are a number of programs, especially games that either have problems or won't run under XP x64. Go to the Microsoft site and look aroung. They've posted lists of software. On the hardware side, Jake is quite right- as long as the manufacturer has released a 64-bit driver, you're okay.
Asking around, I've found that a number of printer manufacturers have decided NOT to write 64-bit drivers for any printer that isn't 'New right NOW' and a couple have said that they won't write ANY 64-bit drivers until XP x64 gets popular (a more assinine attitude I have yet to encounter).
So, don't ASSUME that drivers will show up in few weeks, write to the manufacturers and ASK, if you don't see the drivers already.
<font color=green>****</font color=green> Never Assume <font color=red>ANYTHING</font color=red> <font color=green>****</font color=green>