I'm making my own Camelbak Unbottle. I bought a bladder, slapped together a pouch for it, and am trying to get it to stay reasonably attached in my backpack. Hopefully this will work well, because if the Velcro attachment system fails I'm going to have to come up with something clever fast. Odds are it would simply involve a lot more Velcro, and a bit of swearing. The Instructable.
Update Jan. 2009:
Using a small webbing loop in the top of the bag would be an improvement. The adhesive on the Velcro failed after leaving the pouch out in very chilly weather overnight. Even without the Velcro the pouch helps the bladder stay in position, so the failure was not too inconvenient.