Gaming Halo: Reach's "Challenge Reset" functionality is going to get you nowhere fast, if Bungie catches you. The studio has said via its message board that it is smacking about 15,000 of the "most egregious" abusers of the credit-producing exploit with a credit reset, in addition to a one-day credit ban, though the latter is being done as more of a "we see what you've been doing and you've been punished" tactic.
Cheaters who have evaded the wrist slap this time around shouldn't expect to be as lucky in the future, as an automated banning system will take over as soon as its mechanisms prove accurate enough to satisfy the studio's "high bar for burden of proof." In the words of Bungie staffer "ferrex" on the message board, "If you are thinking about getting an easy 50k credits by using this exploit, I would strongly advise you to reconsider."
As Bungie has clarified in the post, you'll always be allowed to cheese Firefight for extra credits. The bot's AI won't bring down the banhammer on anyone who spends hours on end in "Gruntpocalyspe" or a rockets-only variant or anything. Using Challenge Reset to complete a challenge over 20 times, however? Yeah, that'll cost ya at some points.