Here's an official list of what the PS3 will play out of the box, courtesy of a GameGossip link to IGN.
Supported disc media:
# Super Audio CD (The 40GB Model launched 2007-11-02 does NOT play SACDs!)
Supported software media:
# MPEG-2 (PS,TS)
# H.264/MEPG-4 AVC
# MPEG-4 SP
# ATRAC (.oma .msa .aa3)
# AAC (.3gp .mp4)
# MP3 (.mp3)
# WAV (.wav)
The DVD-R, +R, +RW and CD-R, -RW media are big surprises for a staunch anti-piracy machine like Sony. SACD support is nice to see, too.
MPEG-2 video playback is welcome, as that's a commercial codec right there, and what DVD rips often get written to disk as. MPEG-4 is a no-brainer. They're big on that with the PSP.
Of course I want Ogg Vorbis support with an option for their beta video codec Ogg Theora. The PS3 has far more than enough horsepower to use a floating point decoder, often a sticking point for embedded devices trying to add in Ogg support, and it's free for chrissakes. It'll be in Yellow Dog Linux for the PS3 for sure, but it sure would be nice to have native support for it, especially since games like Unreal Tournament 2004 and later have started encoding game music in Ogg Vorbis to get around licensing fees and get better sound out of lower bitrate files. But I digress (as usual).
I'm also happy to see PNG on the list of supported video formats.