Personally, I think it's worth the effort get a unit that is on the list instead of getting a list of units and seeing if each one is or isn't supported. If you want to eliminate the risk/headache of getting a card that isn't supported, find some way to get the Alfa AWUS036H. Yes, it only supports b/g protocols but it's been proven to work well with Backtrack (or the other way around). That's why most beginners get it.
I was going to get the AWUS036HNR which supports N protocol but I'd read that the driver didn't support injection, which I think is pretty important. But it's a relatively new card and I didn't want to take the chance that it wouldn't work or had bugs. I especially didn't want to waste time searching/waiting for drivers/help if it didn't. So I got the AWUS036H.
It really comes down to what you're willing to accept. If you want to be assured of the unit working with Backtrack 5 right out of the box, get the Alfa. Mine did. If you're willing to accept some risk of a) having to download/install drivers b) searching help forums when it doesn't work c) wait for new drivers or bug fixes for existing drivers d) writing your own, then take a shot with a different unit.