What would indicate a failing axle bearing? I've investigated all sources of a swishing noise coming from my rear wheel and it's leading me to believe that the bearings are on their way out.

loosen axle nut, loosen chain adjuster so that you can remove the chain, retightn the axle, and spin the wheel.  if it is making noise, then bingo  you've found your problem, if not, then I would closely inspect the chain.  if its good, that would leave the rear brake

Stupid question, but have you eliminated the brake rotor as the cause of the noise? Even a teeny warp in the rotor will allow it to hit the pads in places and make noise. The rotor is allowed to have as much as .3mm runout and still be in spec, which should be more than enough to make noise, and that's assuming you're in spec...

I've checked the rotor and ran the bike without rear brakes, same problem.
The sound really amplifies when going a decent amount of speed ~30km/h+. It makes the sound while on the center stand too, and from what I can hear is coming from the rear wheel. I've bought a set of wheel bearings (guy charged me only for one! whooo! 9$ a set!) and am going to work on it in the morning. Just have to figure out a good way to get the old ones out. Might try and take it all apart then re-assemble it first though.