I've always used 260 mah 9.6V rechargeable NIMH batteries, pretty much because when I bought 'em that's the best I could find.
Someone just posted their Deffy frame that had a 9V lithium, so I did a quick google search.
Looks like 9V lithium ion batts are out now - woohoo! They are rechargeable and have a maH ratings of 350, 400, 600, and 1200.
The only glitch is that they may be 'just' slightly larger than 'regular' 9V's..... going to have to dig deeper on that one.