At least in the version we have (haven't upgraded to Tiger yet).

At random points (once a week, maybe? I haven't been keeping track) Safari will start using Lots O' CPU (symptom is the fan on the Mini becomes audible). Shutting down Safari fixes this.

Come to think of it, though, every month or so I have to reboot the Mini due to it coming down with the Beach Ball Flu. Windows take forever to open, programs take forever to start or shutdown, and going to the login screen results in a fetching Dark Blue Screen O' Interminable Waiting with nary a login box to be seen. At this point I have to hard reboot.