Unfortunately I thnk you're right. It really feels lke the Santana situation all over again. Halladay going elsewhere would be okay, Hallady going to the Red Sox would be okay. Halladay going to NY with the Yankees would be very hard for the Sox to overcome, even if they acquired a guy like Mauer for 2011. Wth Teixera, A-Rod, CC and Halladay it would be an enormous hill to climb.
The saving grace--to me at least--is that however much the Jays are reluctant to deal their ace to the Red Sox, I'm sure they're at least as reluctant to deal to the Yankees... probably even more-so.
If I'm the Sox I make a strong offer for Halladay. If it seems like the idea of Toronto dealing in the division is reasonable, then they have to make the deal. It it appears that TOR is overvaluing him within the division then let it go.
If the Yankees really want to deal something like Hughes, Jackson and Montero for Halladay that's not something the Sox could or would easily match.
Underlying all of this is the fact that 2011 is a better FA class. How much would Theo be willing to hold his cards for the next class with guys like Crawford, Halladay, Lee, Mauer and Werth all available?