I'm glad someone else started this thread because I'm on the hunt for a battery pack of some sort!
I'm really eyeing the mophie juice pack air and the incase power slider. The only thing that's keeping me from comitting to one is the thought of a screen protector not being compatible.
OTOH, I havnt completely shut out the option of an external pack as long as it will fit through my otterbox. The gum pack would work perfect for this.
Then there's this...
http://a248.e.akamai.net/7/248/2041/...d=0&layer=comp
So the battery case option would allow you to have the phone cased and ready to charge at anytime and be pocketable or pouchable or whatever. That's a plus for me.
The external dangling pack would be nice to have but you would need to carry it everywhere with you and plus some of those things I wouldn't want dangling loosely from my iPhone to where a bump or something would stress the plastc casing. The plus for me here would be with some I could still use my otterbox.
And the third option you would also have to carry it everywhere with you and hope your near a plug long enough to get a good charge and not just partial charge it all the time possibly messing up the memory of your battery.
The search continues! A lot of good suggestions!