I have used the cart condom as a cover as well. I actually ended up liking it enough that I intentionally removed the ash cap from my other battery and replaced it with about cart condom. I like the extra brightness. I have also see folks use the condoms from Cartomizers as well. Its a slimmer than the 510 cart condoms, but not as sturdy. Plus, now when I accidentally drop my battery they tend to bounce. It seems the cart condom is absorbing most of the shock of falls now.

Kind of an added bonus.
EDIT: Another added benefit... You can blackout some (or all) of the condom with a sharpie to create a design or logo which will glow when the LED lights up...

A coworker has done this with his and its quite amusing.