I think the rate at which the top 10 from last season do NOT make the top 10 the following season is around 50% (if I remember the stats we've done before) so the chances are pretty good that there will be a bust from the top 5. Let's look over who is currently listed as a top 5 pick:
1. Chris Johnson
: There seems to be good reason to be cautious of Johnson and the number of touches he amassed last season. He had 358 rushes and 50 catches which puts him in dangerous levels for a healthy 2010 year (I fully acknowledge that it is impossible to predict injury and that the fabled "370 rule" is flawed to some extent). Tennessee didn't upgrade their WRs in the offseason so teams will be more likely to focus on the running game. Now having said that, I believe one of the big flaws DCs will admit to last season was their idea of *stacking* the line against a player who is faster than everyone else. This means when he breaks through he'll have a free shot at the endzone. I expect this year for them to play a bit further back to limit his big gains. I give him the potential to drop out of the top 5 a 60% chance.
2. Adrian Peterson
: Up until yesterday I would have said the reason he could "bust" is that Brett Favre may not return (even though he put up better numbers without Favre there) so we're going to go a different route: injury. In 2009 Peterson had 365 carries and 46 receptions that went deep into the playoffs. In 2008 he tallied 363 rushes and 21 receptions. That's a lot of touches in two seasons and at some point the Vikings are going to be faced with either reducing his touches to preserve him or potentially risk his health. It's worth noting that Peterson's YPC have gone down in consecutive season now. I believe that is why they drafted Toby Gerhart in the 2nd round of the draft and that he'll get a non-trivial number of touches. But with keeping Peterson fresh it does also allow him to be a bit more effective. Chance of dropping out of the top 5 is 20%.
3. Maurice Jones-Drew
: In MJDs first year as a full time starter he admirably withstood the workload that many were concerned with in the preseason. I think the biggest single concern I have with MJD is the Jaguars and their passing game to keep pressure off their running game. The Jags seem defiant in keeping Garrard as their QB and their WRs are young, developing players. I can see Jones-Drew's YPC going down this year to an unacceptable level which might mean a greater RBBC split with Jennings and possibly Karim. Along with that might be the TDs, which were MJD's bread and butter last season. I give him a 40% chance of dropping out of the top 5.
4. Ray Rice
: Clearly the most unproven of the top 5 backs, he'll need to show that he can handle the load across and entire season. He did take the job and run with it well from about week 8 on, but if I were a betting man I'd put my money here as a near guarantee of slipping out of the top 5. Not that he may drop very far (read: not necessarily a bust, but not in the top 5) but my concern is workload and a potential to fail during the worst time: the FF playoffs. I give him a 80% chance to slipping out of the top 5, although I'd also say it's less than 20% that he'll drop out of the top 10.
5. Michael Turner
: As a fringe top 5 player you automatically get a 60% chance. As a player who has relied heavily on TDs to sustain his position I'd also throw 20% on there. But Turner is one of the most consistent players while on the field. I don't see a reason to believe that last year's injuries were anything more than fluke and then a team trying to rush back their star player. If Matt Ryan doesn't continue to regress as a QB (in fact, I have him for a bounce back year) I have a tough time seeing any more % points going to Turner. If Ryan has a strong year, I would consider taking back 20%. But since we can't be certain of this, Turner stays at 80% chance to fall out of the top 5.
(I just wrote all that up assuming you meant which top 5 RB was going to bust, but realized you could have meant "name 5 RBs that will bust according to their ADP. Hope I got the correct one.