First Off, disclaimer, I stayed on the Drew Bledsoebandwagon, up until the last 1:06 of SB XXXVI, after which, I helped Drew pack his bags.
Cassel right now, is a much better QB, than any of us thought he could be. BUT HE IS STILL NOT TOM BRADY. Brady is still only 30 or 31 years old, has at least another 6 seasons before he even begins to slide. He is signed, and is not going anywhere. In fact the Pats already have his potential replacement (O'Connell) in camp. Give O'Connell, the 4 yearas that Cassel has had and let's see.
Now I can see the argument to trading Brady to upgrade the rest of the team, but I would just not do it. Brady is just too good, and the team "trusts" in him, and if you have ever played football, guys will run through walls for a QB that they know can win the game for them.
As it stands today Cassel is going to net the Pats at the very least a 3rd round comp pick in the 2010 draft. If Cassel continues to play at this level, I think that the prudent thing would be to franchise him, if he signs the tender and they can not move him, they have all the insurance that they would need in case Brady is not healthy next year. More likley is that they trade Cassel to the Vikings, Bears, 49ers for a high draft pick(s) and all this can be done without any salary cap hits, which any trade of Brady would include.