Warning: This glitch has been reported as patched by our community. While this report may be a false-positive, if you absolutely cannot re-create the glitch, it is more than likely true that it's patched.

Download the attatched file. Unzip, make backups, w/e.

Download File Here

Get your mpdata from your HDD/Mem card.
Make a copy of it.
Inject the Error mpdata into one of them, the prestige into the other.
Put ERROR on the mem card, and PRESTIGE on the HDD.
Start up MW2, have HDD and mem card plugged in.
Hit multiplayer, set device as mem card.
Hit XBOX LIVE.
Wait for your stats to DL.
Hit B.
Hit SPLITSCREEN.
Hit A.
It should error you out.
Hit A until you are in the splitscreen lobby.
Go prestige once.
Hit B.
Hit Y.
Chose HDD.
Hit A till the splitscreen lobby.
Hit B till you are at the Main Menu for multiplayer.
Hit XBOX LIVE.
Find a match.
Rank up once.
???
Profit.

BTW, I just saved you $100.