I just ordered an Apple TV this morning. First thing I plan to do and you should too is buy and install this:
http://www.atvflash.com/index.php
You'll need a memory stick and about 30 minutes but this little flash will unlock all the power apple should have let you have from the factory. You can play almost any format, surf the web, and unlock the USB port to allow more storage through an external hard drive.
This mod most likely voids the warranty but even if it breaks they are not that expensive.
ATV + aTv Flash = hands down winner
OBTW - in response to a previous post alluding to the ATV being hard to navigate is outlandish. ATV is based around OS 10 making it highly reliable and the menus work just like Front Row, Its dummy proof. IMHO Windows Media Center blows