ZodTTD, creator of GBA, SNES, PSX, and OpenTTD emulators, has successfully ported VLC
to the iPhone. VLC is a cross-platform media player which supports many formats - far more than the iPhone's built in media player does.
He's still working on some of the formats, but the beta is up and available to his donating beta testers
. Expect a public release sometime in the next week.