Separate from the issue of whether the U.S. should send more troops to Afghanistan, Adam raises the question of whether domestic political priorities are the real drivers behind both presidential candidates' support of doing so. I'll give you John McCain, whose decision to support Afghanistan troop increases seemed more an attempt to prevent Obama from outflanking him to the right than anything else (unless he withdraws from Iraq he won't even have the troops to do it).
But I do think its unfair to suggest that Obama is pulling a 2004-era Democrat move and trying to show he can be hawkish when he wants to be for some political gain. One, I think it's fair to say that Obama and his advisers honestly believe that their policy is the right one. Two, most people are ambivalent about the Afghanistan conflict; it wouldn't be hard a sell to make the argument that more troops are not the answer -- especially when there is majority in favor of pulling out of Iraq. Obama could have likely made a politically strong argument that the need for more troops in Afghanistan isn't there, but he didn't. Instead, he got out in front of McCain with the "strategic redeployment" argument based on his recognition of the true priorities of national security.
So is an increased military deployment inevitable? In one sense, yes, since both presidential candidates have promised it. On the other hand, no, because McCain won't have the resources to do it and Obama and more likely, his advisers could respond to, yes, the cottage industry of skeptics in the foreign policy community (Juan Cole, Zbigniew Brzezinski, Robert Kaplan, Rory Stewart, etc.) and also whatever the situation on the ground is in 2009.