I think this is a very smart move. I think Deion is going to be VERY surpirsed at the lack of offers he will receive. I find it very hard to believe that he will attract a top $ contract, and that team will also pony up what the Pats would want in trade.
This is a risky move however. If Branch does find a suitor, and they lowball the trade offer, he will try to hold the Pats hostage. (although he has few cards to play, we can just let him sit, the year gets wiped out, or if he reports on his current deal he will still hold the franchise option)
Secondly, having a firm offer could outprice the Pats if someone does give him crazy $.
Thirdly, this will probably reduce the chances of Branch resigning here before going into FA assuming no deal can get done, and he plays out his contract (unless we tag him).
A shrewd team could offer him a ridiculous contract, then fail to give the trade bait, in order for him to think he can get more on the open market and refuse to resign here. If Mangini learned from the master, he will offer Branch everything he asked for, then offer the Pats a 4th for the trade, meaning there will be no trade, and Branchs future here is over.
For 2006, this move is a good one, for beyond, its very risky.