Even Tom Curran said yesterday that there is no way Brady walks away from the game.
With all due respect to Mr. Curran (and yourself), there is only one person that can know that for sure. I don't think this is simply a matter of wanting to walk away or not. I think if Cassel was still under contract in 2009 (at his present cap figure) then this wouldn't even be up for discussion. Brady would get himself to 100% and Cassel would be the #1 until Tom returns (assuming it's not December '09).
Cassel is the wild-card in this whole situation and him proving his worth this season is probably on Brady's mind. I don't think Tom would want to leave the Pats holding the bag at the QB position. If Tom wasn't certain he could play the majority of 2009 at a high-level then he wouldn't leave the Pats with O'Connell (unless KOC is the heir-apparent, which is a whole different discussion).
Cassel represents a proven commodity in the Pats' system. His numbers this year aren't disimilar from Brady's averages from his first 3 seasons (which represent 4 years in the system for both QB's). Of course Cassel does not have the playoff pedigree or can even approach Brady circa '07 but at this point even Brady isn't assured of reaching that level again.