I have no idea what Barack Obama will do at tomorrow's meeting with Stanley McChrystal. On what I think he should do, see here.
But to complete a thought not addressed earlier: the strongest argument against Obama's firing McChrystal, or accepting his resignation, is that the American strategy in Afghanistan is inseparable from McChrystal the commander. Since Obama has gone all-out behind this strategy, he cannot afford to displace the general who was central to its design and execution.
First, the textbook point: if national strategy for a war rises or falls on one officer, that's a bad sign for the strategy. Banal point, but had to say it.
Relevant point when it comes to the counterinsurgency (COIN) strategy in Afghanistan: two years after the invasion of Iraq, Mattis was a leader, with David Petraeus, in the effort to develop a new military approach based on awareness of everything that had gone wrong in the invasion and its immediate aftermath. For accounts of that effort, see here and here. For the resulting Field Manual 3-24, the doctrinal Bible of COIN, you can get the 13MB PDF here.
Relevant point more generally: I have never met or interviewed Stanley McChrystal, but I have interviewed Mattis repeatedly over the years and have always been impressed by his intelligence and character. Tom Ricks testifies to that effect here. While he has made waves for his blunt-spokenness, it is hard for me to imagine him being as careless as McChrystal seems recently to have been.
Marc Ambinder says just now that Mattis is not likely to be interested in the job. (Spencer Ackerman on this theme, here.) Moreover, there's an argument -- which I unfortunately believe -- that the job and the strategy are themselves doomed. Still, here's the point: If the argument against firing McChrystal is that he's the only man for the job, that's not good enough. There's at least one other four-star general perfectly well prepared.