@panda
Video playback from iPods is not possible
Have not tired from a usb device but I highly doubt it would work.
Any mp3 player that supports the mass storage profile (ie. it appears in my computer as a disk) will work with the usb interface.
USB hard drives also work, but power is an issue - the usb bus can only offer a limited amount of power (~5v @ 100mA is the spec, not sure what is actually coming down the cable), so usb disks that are bus powered (no ac adapter) are unlikely to work, although they may if you are using a decent cable - if one dosn't work first try find the fattest, most over-engineered cable you can and give that a try, worked for me. If you are desparate for bumper storage you could always lash up a usb disk with an external power adapter to the 12v cig lighter socket (only if you know what you are doing vis. polarity, voltage, etc, dont go plugging any old adapter in!), that would work a treat.
I can confirm that the usb interface supports SDHC card readers and cards of at least 8gb, and very likely 16 and 32 aswell. I havnt tried a SDHC in the slot on the head unit, will see if that supports it tomorrow (IIRC there was some contradictory comments in the handbook).
Peace
[update]
Can confirm that the SD slot on the head unit plays SDHC cards fine (I tested using an 8gb sandisk microSDHC card through a sandisk microSD->SD adapter).
Also noticed that it does
not support AAC with M4A encapsulation (iTunes format)