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Check the XBMC forums. Basically you swap out the mini PCI-e wifi card with the Broadcom card, then use the USB ATV flash approach to install XBMC (and other apps) onto the ATV.
 
Thanks for the replies, I guess its pretty simple? I've built a PC and I have some basic knowledge of stuff so I should be able to do it I think.
 
Taking the rubber part off the bottom is the toughest part. One tip is to plug in the Apple TV and let it run for 30 mins. It will warm up nicely and the rubber pulls off easier when its warm.

From the launcher menu, you can install the crystal broadcom drivers. If you search the XBMC forums their is a how to on getting the "download" broadcom drivers added to the launcher menu. Just requires ssh to the apple tv and pasting the entry at the terminal.
 
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