Jason is keeping track of Mac OS X apps that may or may not need upgrades with Mac OS X 10.4 Tiger, which we Mac loyalists (that aren't Mac developers) will be getting very soon now. This will be my first major OS X upgrade since I got a Powerbook, so I don't know how reliable an upgrade (in-place) installation is expected to be. This may be the first time since I ran MS Windows that I'll backup, catalog, and disassemble everything on my hard drive, wipe the disk and install everything from scratch, but I'm pleased at the thought that it may not be necessary to do so. (I'll do it anyway, naturally, because it's one of those compulsive things I like to do.)
Derrick Story has an article about preparing your Mac for a clean (backup-wipe-install) Tiger upgrade.
My Tibook has been upgraded in place since 10.1 or so. (January 2002, whatever was current then). I'd generally say that it's safe to just wing it and do the in place upgrade. Just make sure that you've removed any special kernel extensions before you do, since one of them stopped me cold when running panther the first time.