We have a G4 iBook running 10.3.9 that has been running extremely sluggish and very hot -- too hot to rest your hand on the lower left corner. A visit to the Activity Monitor confirmed 100% CPU usage, and the culprit was a process named fsck_msdos. This had me confused: I know what fsck does (it verifies and fixes disk volumes), but I do not have a FAT-formatted volume connected.
Further experimentation led me to a defective CF card. Apparently, when diskarbitrationd attempts to mount the volume, it launches fsck_msdos. And fsck_msdos appears to be brain-dead: if it cannot access the volume, it just sits there and consumes CPU cycles...all of them, as long as the machine is on, whether or not you remove the defective card.
The solution is to simply kill fsck_msdos in the Activity Monitor, and read the defective card through the camera's USB interface. Surprisingly, a search of the Apple forums in the 10.3 discussion threads produced exactly one hit, which had nothing to do with the problem.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071202140437671