This is a little Cheaper way to get a battery back for people in the US.
Only need 4 items from Radio Shack
Battery Holder
http://www.radioshack.com/product.as...5Fid=270%2D383
AC-DC Adaptor Cord.
http://www.radioshack.com/product.as...Fid=273%2D1641
Snap Connector For the Battery Holder
http://www.radioshack.com/product.as...5Fid=270%2D325
Adaptaplug Size H
http://www.radioshack.com/product.as...Fid=273%2D1711
Now Granted it isn't a nice neat box with a power switch, but it will do the same job as the others and a little easier on the wallet after the $400.00 Iriver price tag..
I am not going to go into detail how to hook it up it is more or less self explanatory, but if someone has some trouble I will help out..
I ended up using 4 AA Rechargables at 1.25v each to give me the 5v's charge light comes on when it charges as far as what kinda of play time I have nothing to go on since I just put this together last night... I hope this helps out someone else other then me. since the battery boxes the UK guys were using can't be found(at least I can't find them yet) here in the US..