Several Longhorns were drafted or were picked up as undrafted free agents, but I'll just focus on the ones that were drafted because they aren't in danger of being cut (Marcus Griffin, for instance, was cut before July).  The four drafted were Limas Sweed, Jamaal Charles, Jermichael Finley, and Tony Hills.  Of the four, I think Limas Sweed will have the biggest impact his rookie year, and I'm sure most people would agree.

Sweed is coming off a wrist injury, but he seems fine and I really think he landed in a good place.  Many people were a bit annoyed that Tennessee didn't grab him to help Vince, and while I too would have liked to see VY and Sweed together again, going to Pittsburgh is probably better for Sweed.  I also think the Steelers got a steal, because my biased self thinks that Sweed was the best all-around receiver in the draft, possessing the best combination of speed, size, leaping ability, hands, and agility.  He is just as fast or faster than the receivers his size, such as Malcom Kelly, and bigger than the receivers faster than him, such as Desean Jackson.  While bad for Texas, the Steelers are probably happy that Sweed got hurt and subsequently dropped to them late in Round 2, but I digress...

At any rate, Sweed will not have to be thrown into the fire and will have the luxury of easing on in.  If Willie Parker can rebound from his injury, the Steelers will once again sport a strong running game, which is now complemented by Mendenhall, a threat that opposing defenses must respect first and foremost.  That leaves the passing game with a bit more space to work with, led by Roethlisberger, Hines Ward, and Santonio Holmes.  Thus, Sweed will have little pressure on him in the beginning and can present mismatches because of his size and speed, especially since teams will have no choice but to pay close attention to Holmes and Ward.  I think Big Ben is absolutely giddy with the weapons he has:  He has a run game with Willie Parker and Mendenhall and pass targets that include Ward, Holmes, and now Sweed (and Miller isn't a bad tight end).  I expect Sweed to begin to emerge by midseason as a redzone threat that will strain opposing defenses who have a several other players to worry about.  He can also cause headaches if opposing teams are forced to grant him single coverage with their third or fourth corners.

Basically, he's in a situation where he's playing with a Pro Bowl quarterback with a big arm, a running game that will keep the safeties honest, and a passing game where other receivers are featured and will attract the attention.  He may not put up gaudy, fantasy friendly numbers, but I think he'll present the big target Big Ben wants and contribute nicely.

As for the others, Charles seems to have grabbed himself the #2 spot and will receive touches as LJ's complement.  He also will contribute on special teams.  Finley may fit in the Packers pass attack nicely, but he doesn't have Favre throwing to him and he still needs to work on his blocking if he wants to see the field more often.  I have no idea about Tony Hills; I haven't seen him much at all.

In any case, I'm not just saying Sweed because he happened to be picked first.  I think he simply landed in a great spot for his skills to develop.  However, I am sure there are some people who feel that either JC or Finley will have the more impactful rookie season, so I'd love to hear from them.  Charles has already received rave reviews on his speed and quickness and, because he's currently the return man for Kansas City, may receive a whole lot more touches than Sweed.  One could make a good argument that him touching the ball 10-15 times a game as the #2 back and as the returner is much more impactful than Sweed's role.  Maybe so.  In any case, I hope both have good years.

edit:  By the way, I am aware the Charles in injured but I don't know how serious it is.  Will be ready by the season?  Obviously, he can't be in this discussion if he isn't playing.