Grant and Jackson are really the only "locks" I'd consider out of group of RBs we'll be taking in to training camp to make the final 53-man roster. Grant because of his production last year and Jackson due to how he came on late in the year and was a second round draft pick in '07.
Wynn, Morency, Herron, and Lumpkin are all bubble guys who are going to have to earn that final spot (assuming we carry only 3 RBs on the final 53-man roster) in training camp. The only reason we carried 4 RBs into Week 1 on the roster last year was mainly due to injury or inexperience with the system in Grant's case. Lumpkin and Wynn are still both practice squad eligible while Morency and Herron are pretty much playing for their football lives due to their number of years in the league.
Wynn showed some flashes early in the '07 season, but never did enough to really lock the job down. The fact he's injury prone and his work ethic are still big question marks. Him coming into OTA's in "so-so" shape is just another idication of that in my mind. Morency is another guy who has shown a flash or two here or there since getting traded to Green Bay. He seems like a relatively pedestrian change of pace back, but has made some nice catches out of the backfield. Like Wynn though, he's got some injury issues too in his past, but if he can stay healthy through OTAs and training camp, I like his chances best out of any of the remaining RBs to make the final 53-man roster. Herron has been a hard worker and did some good things for this team in '05 and '06, but he's never shown really great flashes at any time and is probably on the outside looking in come the end of camp unless he really shows some things in camp. Lumpkin I can't judge because we haven't seen any of him yet, but he seems like a sure-fire candidate for the practice squad if he has a good training camp.