looks like they are adding one feature the normal PS3 can't do
http://www.blu-ray.com/news/?id=3301
the slim is getting a different HDMI chip that can bitstream Dolby Digital Plus, Dolby TrueHD, and DTS-HD
no actual difference in sound quality between bitstreaming and decoding on the player (and ouputting as PCM) but people with receivers that can decode those formats will get to see the indicator lights for those formats light up with the slim
the only real advantage here is it would mean discs with regular DTS and dolby digital tracks (both DVD and bluray) will not need you to switch audio settings from PCM to bitstream in the event of a track that isn't plain stereo or 5.1ch
examples
using PCM output mono tracks get sent as stereo instead of proper mono (which should be played from the center channel speaker) dolby surround tracks carry flags that tell the receiver to use it's pro logic mode that are lost when decoded on the player and you have to manually activate pro logic mode on the receiver (no big deal but mildly annoying)
6.1ch tracks get sent as 5.1ch... you have to change the audio mode to bitstream to actually get 6.1ch
they could still add an "auto" mode to the fat PS3 that automatically selects bitstreaming for regular DD and DTS and PCM for everything else but it seems such conveniences to the customer are too hard for sony to think about