I think you have to search for it; go to XBLA and search for "Black Lavel"
It's $15 and allows you to play the Black Label version of the game, which IMO turns Futari into the most perfect shooter ever made. There are a number of tweaks for Original, Maniac, and Ultra (God)
Basically they made each of the game modes slightly easier, but put in a lot of things that give them ridiculous scoring potential. There's also slightly different enemy arrangements, patterns, and palette swaps. If you like playing Futari for score, it's a must have.