It's a tough decision for the Colts (if they get the pick).
Bradford and Ponder are young, so they don't have as much worry about 5 years down the road. The Colts have to worry about how much longer Peyton will play. If they only get 3 more good years out of him, are they better off with a successor? Should they draft Luck, groom him, and trade Peyton a few years from now?
It's tough to make that call with a future hall of fame QB on your team. The Packers had a similar situation with Favre and Rodgers, except I think Favre was older. Peyton has the threat of future injury looming.
If I were the Colts, I would guage trade interest for the pick. See how much they can get for it and how far down they would drop in the draft. Unfortunately for them, Barkley is not coming out. More first round worthy QBs would be a benefit.
The Rams, Vikings, Buccs, and Browns all seem to have their QB. The Browns might value Luck over Colt McCoy. The Jags also might feel like they are set with Gabbert.
The Bills, Redskins, and Dolphins would all probably covet Luck.
Could the Colts trade down with the Bills or Redskins (whoever picks first), give them the Pick for Luck, and still be able to draft say, RGIII. I think having RGIII learn for a few years under Peyton would actually be ideal for him rather than a hindrence for Luck.
If Landry Jones comes out (and I hope both RGIII and Jones come out) the Colts to drop further, get more picks and take him later in the first.
The Colts would be smart to draft a very good successory for Manning. they have shown this year how good they are without a good QB. If they could trade down and secure multiple first round picks it might be the best thing for the organization. They obviously need more than one piece if they fall apart this much without Peyton.