Will OSX read/write to an exFAT external drive?

Will OSX Tiger read/write to an exFAT external firewire drive formated on a Windows Vista machine?

NO is the simple answer.
exFAT is a fairly new filesystem and it's not what they call "open" so it's unlikely to be compatible for quite sometime. That said however an NTFS formatted drive is easily usable with tools like the one from Paragon.
check out ntfs-mac here

mmm... thought not. The Paragon software looks interesting, or I might go the other way and get MacDrive for the Vista machine. Thanks for the reply.

No problem, you're very welcome.
MacDrive is a very good application. It makes Mac formatted drives available to you PC whereas the Paragon Software does it the other way around. Making PC formatted drives available to your Mac.