Assuming Tom Brady is back in the Spring and his rehab is preceding well, then I say the Patriot's can tag Matt Cassel. If Brady isn't back, then they will definitely tag Cassel.
It's not a foregone conclusion that Cassel would sign the franchise tag right away. Look at the going rate for backup QBs right now. It's $4 million a year. If the franchise tag is $14 million, then Cassel and his agent can talk to the Patriots about a sign and trade. If another team is willing to give him a $10 million bonus (which isn't out of the realm of possibility) then Cassel might prefer a guaranteed $10 over a guaranteed $14 IF IT MEANS HE'LL BE STARTING instead of sitting the bench. Getting $14 million from the Patriots so he can sit behind Brady is not as good a career decision as moving elsewhere, starting, and getting a reasonable longer term contract for a mid-level type QB.
Look at Derek Anderson, please. Anderson is both the reason why Cassel would get a big contract, AND the reason that a team won't overspend for Cassel, since both could be considered one year wonders.
Again, my only point is that Cassel would NOT necessarily sign the franchise papers right away, which might give the Patriots ample opportunity to trade him away for a 2nd and 3rd rounder.