Yea I was wondering this too. This last year was my first dynasty league season. My league is 12 teams 1pt for Tackles and 1pt for Assists. I think I'm pretty solid at linebacker and DE, but I was interested in taking Laron Landry or Chris Houston to help out at CB and Safety. How soon in a rookie draft should I try to pick them?
I realize that things differ from league to league but in my experience (two start 3 DL, 4 LB, 4 DB dynasty leagues going into their eighth years) it is extremely rare to find a DB that lives up to first round value. We do not seperate CB and S but I have always found DB to be the easiest position to fill on the waiver wire. These are deep leagues (one 12 and one 16 teamer) with deep rosters (45 players) and there are ALWAYS viable DBs on the wire.Even DL are too volatile to predict, although the better ones are more scarce overall so I would be more inclined to take a shot at a Gaines Adams than a Laron Landry.
I usually find that there are a couple LBs that are worthy of first round selections. Last year AJ Hawk went 1.10 and Bobby Carpenter went 1.11 (to a huge Dallas homer) in my 16 teamer. Hawk was worth a first round pick and so were Demeco Ryans (2.10) and Ernie Sims (2.04). Of course, Chad Greenway (2.03) and Manny Lawson (2.11) were picked in their vicinity. LBs are the ones that usually have the most value for me when the dust settles...
you just have to make sure you take the right ones.
Good luck!

I agree on the DBs, the only one I remember thinking I would take late in the 1st was Polamalu, he's pretty much lived up to it. DL usually aren't worth a 1st either.
I used to think I'd never take a LB in the 1st, but I almost took Hawk there last year - I didn't, ended up with Jennings instead, but he's the only LB I came close to taking in the 1st.
In a redraft, IDPs aren't hard to grab off waivers, but in a dynasty league, getting a top LB can be better than grabbing a WR prospect. Generally speaking though, you can trade a late 1st for a good 28-30 year ol LB, which might be the better move.
I would not take a corner in the 1st, probably not in the late 2nd either. In my leagues, corners and DTs just aren't in high demand, many teams would rather grab one off waivers and any points they score are considered a bonus. I actually traded for Kevin Williams and Dunta Robinson last year, didn't have to give up too much.
Just as an example of Values, in a trade made before the season:
OZ gave up
Brown, Reggie PHI WR;Suggs, Terrell BAL DE;Parrish, Tony DAL S; Year 2007 Round 1 Draft Pick (ended up being #8); Year 2007 Round 1 Draft Pick (ended up #16)
for:
Walker, Javon DEN WR;Kinney, Erron TEN TE;Williams, Kevin MIN DT;Umenyiora, Osi NYG DE;Polamalu, Troy PIT S
Back in April, I gave TO for Hines Ward and Dunta Robinson. (this probably wasn't a great trade, but I liked Ward before Ben's accident)