On Mac OS X, there are lots of very good reasons why syslogd could start eating 100% of your CPU. Plenty of discussions about this on the net out there.

Well, here's one more (very good) reason:

If all of a sudden, your Time Machine backup process appears to be slower than usual, and if the backup disk is not unmounted after a backup has finished, it could be that Spotlight is trying to index it. Stupid. Mine currently has an ETA of 55 hours :-)