mophie makes a couple for the iphone5 - i have the mophie air - works fine but a little hard to get the phone out of the case since syncing doesnt work with the case (only charging - according to Mophie) - it uses the lightning connector- there are lots of others
ivyskin makes a case for the iphone5 with a detachable battery - so you only have the extra weight when you have the battery attached - it also has a buitin screen protector
www.ivyskin.com
i had one for my 4S - pretty good case but the company has lousy customer service
a company called Third Rail Mobility (www.thirdrailmobility) has a similar case for the 4S and is workin on one for the iPhone 5 but i dont think its available yet
Also a search in Amazon for iphone 5 battery cases will show lots of others