Sure, the Philadelphia Eagles will ultimately determine whether to trade Kolb, and for what price. But Kolb will also wield influence over the process through negotiations on a new contract. After all, any team acquiring Kolb will want to work out a long-term contract with him. Kolb will have to approve any new contract.
Kolb's deal with the Eagles runs through the 2011 season only. If the Cardinals are as serious about landing him as reports suggest, they'll want to work out a long-term deal with him to facilitate a trade.
Arizona or another team could, in theory, acquire Kolb under terms of the quarterback's current deal. That would not make sense, however. The Eagles wouldn't receive as much in return from a team uncertain about its ability to sign Kolb for the long term. The Cardinals wouldn't want to worry about new deals for their quarterback and their best player, Larry Fitzgerald, at the same time.
Seattle showed interest in Kolb last offseason. The Seahawks' could again show interest when free agency finally opens. If nothing else, they could drive up the price for a division rival, something the Cardinals did, in effect, by showing interest in acquiring Charlie Whitehurst from San Diego last offseason.