Everything is date-named. I use a leading "P" (for picture) partly so the filename isn't all-numeric and mostly for historical reasons. Then the date and time come in yyyy.mm.dd-hh.mm.ss format so that pictures will sort in date order. Then a 4-digit image number that's directly from the camera sequence. Finally, "-bw", "-hdr", "-stitched", "-processed", "-forweb", "-cropped", "-forweb", etc. for stuff that I've postprocessed. So in the end I'll have something like P2010.08.23-14.32.20-4392.cr2 which gets processed to P2010.08.23-14.32.20-4392-bw.jpg.
The original purpose of date-naming was to let me look things up by date (I often have a good idea of when the shot I want was taken). The other advantage is that it guarantees me unique names for every file; none of this garbage of having "IMG_0001.JPG" reappear every 10K clicks.