It depends what kind of off-field behavior it is. Everyone loves Mickey Mantle even though he was an alcoholic, but Michael Vick is shunned.
Vick is still the man and he got screwed because he's black and he was a superstar. If it was discovered that a mediocre white NFL player was into dog fighting it wouldn't have been nearly as big of a deal.
This site has 7 pages of arrests of just NFL players and I doubt their memorabilia value has diminished at all.
http://www.signonsandiego.com/sports/nf ... PIorderBy=
It's definitely more common for NFL players to be arrested (there are more of them) but there are MLB players like Brett Meyers (Phillies) punching his wife on a Boston street, and Elijah Dukes (Rays) threatening to kill his wife and her kids, even sending her a picture of a handgun.
I don't understand it, but if you get caught for somethings (animal cruelty or steroid use) you're going to get buried, but if you beat a pregnant women or run someone over with your vehicle, you're usually fine.