I agree with your assessment that DB help is needed as I do not feel comfortable with only having three truely competent corners on the team right now, and I would guess most people would like some type of linebacker youth infusion. However if the Patriots were to get the #2 and use it on Jake Long or Darren McFadden, that presents a problem. Yes both players are most likely an upgrade over the current holders of the position [Long moves Light to the right tackle position, upgrading LT and RT by at least a marginal amount, and McFadden v Maroney probably is an upgrade]; however the problem is that the advantage of good drafting is that you should be able to get above average talent for below market rates for four to six years.
See the deal that we effectively got for our D-line, or Samuels compared to their second contracts. That divergence between cash and cap outlays v. true market value has allowed the Pats to allocate their cap to address other needs. Picking at #2 removes this projected divergence unless you believe that anyone the Pats pick there will have multiple All-Pro years in their rookie contract AND be a Pro-Bowler in their rookie year.
So if the Pats were able to move back to #8 and pick up say #40 and at least one first day pick in 2009, the Pats most likely will still improve their overall talent level for the 2008 season while providing for cheap depth [#40 has similiar cap charges as a vested vetern brought in on a league min. deal in 2008; see the Chad Jackson contract on patscap.com for the details]. Note that I have the Patriots significantly underselling the #2 pick to avoid salary cap charges in future years --- the DVC has the @2 at 2600 points, and a trade of #8, #40 and 2009 1st Rounder is only about 2400 points.
In my mind if the Pats have a choice of picking Long @#2 and then nothing until #64 and 66/67 [Raiders], or picking the #2 OT at #8, and the #4 or #5 CB at #40 and then filling other needs in the 60s, I'll choise the second scenario, for although there is a drop-off between Long and the #2 OT, I think the team is improved more and at a lesser cost by getting the #2 OT to effectively replace Kaczur, and getting a legit potential starting CB who should be able to displace Gay as the nickelback allowing Gay to move to the #2 position if Samuels leaves.