Defensive linemen are the players least likely to produce a rookie with a fantasy impact, but there are still a few guys to keep an eye on. None of them are worth drafting in your league unless you're in a very large league (14+ teams) or you use a large number of defensive lineman but they could be good waiver wire pickups.
Gaines Adams (4th overall) - Adams is a great talent, but unfortunately he was drafted by the Buccaneers who already have a surplus of quality defensive ends. With a (supposedly) healthy Simeon Rice and newly signed free agent Kevin Carter, it will be hard for Adams to find playing time. If he does, his speed and quickness is perfect for the "Tampa-2" defense. Unless somebody gets hurt you'll probably never see him, but he could be worth a one week injury fill-in.
Jamaal Anderson (8th overall) - In contrast to Adams, Anderson was drafted by a team that rolled out the red carpet for him to walk straight to the field. The Falcons drafted Anderson to fill the hole left by the departure of Patrick Kerney, so Anderson will be playing from the first snap of the season. He's considered a much better pass rusher than a run defender, so I expect to see the Falcons use him where he's strongest.
Anthony Spencer (26th overall) - Spencer came out of the same Purdue program that produced such fantasy stars as Akin Ayodele, Rosevelt Colvin, Chike Okeafor and Shaun Phillips. They don't win a lot of games in Purdue, but they sure know how to produce defenders. Spencer should see playing time right away and should make an immediate contribution.