Well, Leopard is installed and working great on a Powerbook G4.
Straight from 10.3, no problems.

In anticipation of Time Machine, we bought a HUGE external drive and
mounted it in a simple media player case...plug it into a tv and it
will play movies, plug it into a mac via usb, and it'll be a normal
drive. It's great 'cos we've got all the dvds and tunes on it and made
a simple zip backup before updating to Leopard. However, we plugged it
in last night ... "this drive needs formatting".
Well, that's not going to happen...not when it took about 2 days solid
to put our dvds on it. And besides, the media player needs fat32 only.
So, my question is (and I can't be alone here).....
As a bit of a "mac newbie", is there a safe and simple way to make a
partition in the spare 40Gb of the 320Gb drive that Time Machine will
like, but that won't confuse the media player case?

I've read various things about an early bug which prevents Time Machine backups > 10Gb on MBR disks, eg:
I haven't seen that it's been fixed yet, but I'm pretty sure the Media Player case needs MBR to boot - it has to be FAT32.
Plus, I can't find a quick and reliable guide to repartitioning and reformatting the right partition in the Apple Journaling file system, so the Mac will see that partition, and then the media player will only see the fat32 partition to boot off.
This is the media player:

And finally, if it's all going to be too much trouble, is a standard backup of dragging files across OK if the FileVault is turned on?