I've started freezing my camelbak, too. I fill it between half and two thirds full. When I'm ready to use it, I run it under tempid water for a couple of minutes (seconds -- just until I can open it), and that thaws it enough to get the lid open and release the bladder from sticking to the ice. Then I top it off with water until it's full. This works pretty good. Running it under the faucet thaws it just enough so that the ice can shift and is not blocking the outlet.

The suggestion about blowing air into it so it freezes puffed up sounds pretty good -- I think I'll try that
HTH,
Christy