if you buy a title from the xbl games on demand service, can you re-download it if something happens to the drive it was stored on?  i'd assumed so because that's how other XBL content works, but i've seen people on the internet implying otherwise.

I'd like to download God and then stomp his balls to death

yeah... 
 
no, seriously.

It should be linked to your gamertag, so as long as you're logged in you should be able to download it. You should also be able to transfer the license if you get a new system, I'd imagine.

i should've just googled it to begin with, but in case someone else stumbles into this thread wondering what i was wondering:  
 
Q: What if something happens to my hard drive or I run out of space? Can I re-download the games I have purchased?
A: Yes. As with other Xbox LIVE content, your game license is linked to your Xbox LIVE account, so you can re-download anything you have already purchased to the same console or another console. 

^From here.