Not sure how they're going to defrag it exactly, but look at it this way: You had to be pretty fucked up to NOT get selected for Captain before there was a shortage and before there was such a thing as a plus-up. If you have a heartbeat, don't lick windows on a regular basis, and have not committed a crime, you will pretty much get promoted. The selection rate for FY08 was something like 99.5%, including above zone people and those who requested not to be considered.
If you're thinking of career designation, however, that's a different story. It's today's answer to augmentation, and it's a separate board that meets around the begining of each year. The pain about this board is, even though you might want to accept career designation, if you attend flight school, you're probably getting unobserved FITREPs, and you need a minimum of 540 days of observed time to even be eligible for the board. So people like me, who attrited and redesignated into a new MOS can't get considered until later. But again, if you've got a heartbeat and you haven't been caught beating your wife half to death, you're probably going to be selected. You do have to either accept or deny it, though, and if you accept career designation, you incur a minimum two year extension. I'm not sure how that works for aviator commitments, though.