Why is my wired keyboard's Eject key no longer working after OS upgrade?
In response to Allan's comments on the original question, I attempted safe mode and found that the eject key still worked and also realized that it worked before I logged in to my account. Obviously, it was something my profile was loading at login time.

While I'm not sure which item specifically helped, I did notice that I had an older "iTunes Helper" and also quitting iTunes (which is normally loaded all the time) did seem to correspond with the eject key working again, but I'm not 100% convinced that it was the precise culprit. While iTunes has always had a helper, I think this particular one might have been an artifact of an older installation that got carried over in the upgrade and caused the problem.