There are also the type that's an external battery pack with a cable:
http://www.provantage.com/apc-upb10~7AMP90J5.htm
http://www.imaxpower.com/
By the way, there's a good thread on this over here:
http://www.howardforums.com/showthread.php?t=1200682
I'm personally liking the Mophie Juice pack design. It's not something I'd have on my phone all of the time. Probably just to charge it while mobile or watching movies on a plane, etc. But I can see the advantages being:
A) You could slap it in for an entire day and deal with the bulk if you knew you'd have a long day (like at a trade show, a lot of traveling, etc.)
B) Might be less conspicuous going through airport security. ??
C) Offers case-like protection. Obviously you'll have to take it out of the case to put this on and this wraps it well.