This is just and educated guess, so take it as such.
- MP3 support is a given, it would be suicide not to. (I'm not sure who's picking up the licensing cost for the codec though... probably the hardware manufacturer)
- I expect that Google docs will run rather well on Android, so anything you can open with google docs, you can probably open with android.
- PDF... that will come. Someone will write an app for that quickly. Until that time I know gmail always offers to show me pdf attachments in html; it doesn't work very well but it's sufficient in a pinch.
I have no idea about video... I know it will support video to some extent (it has to, right?) but I think actual codec support is anyone's guess at this point.