If you're considering installing Mac OS X 10.3, and you're planning to back up your data on an external Firewire hard drive first, be aware that there've been a fair number of problems reported, ranging from the new OS not recognizing the drive to (in some cases) actually corrupting the data on the drive.
Nobody's quite sure what causes this. Apple says it happens only with newer Firewire drives that use a particular chipset, but someone from Macfixit says (as quoted in a Wired article) that it can happen with any Firewire drive that's connected to the Mac during 10.3 installation. There's been an awful lot of traffic about this on an Apple discussion forum.
I didn't have any problem; my external FW disk is still working fine. (For what it's worth, I didn't have it connected to my Mac during installation.) But it's still a little unclear what exactly the cause of the problem is, so I recommend being extra-careful until Apple's done some further investigation.