It is all supply and demand. Even if he was universally rated a top 10 QB for 2009, that doesn't mean there are 22 teams willing to trade for him. Only Titans, Jets, Chiefs, Lions, Vikings, Panthers, Bucs, Redskins, Niners and Rams are even viable options. The Titans, Panthers, Redskins and Rams seem to be willing to go with what they have in 2009. Nobody is trading for Cassel to sit in 2009 and become the franchise QB in 2010 or beyond. That leaves only 6 teams possibly in the bidding.
That is still a healthy market to get good value in a trade, but start knocking a couple off (Jets would never offer enough, Chiefs and Lions look long-term with Stafford and Sanchez, Vikings trade for Sage) and now things get a little dicey with only two bidders. Drop one more team and the trade value nosedives. Lose the last team and goodbye salary cap (and maybe Wilfork).
If the Pats have a deal in place, all of this is moot. If they don't, there is significant risk this situation could end up less than optimal.