@P_Wood: I believe that I've seen a few projects that use squishy buttons like that to depress tactile switches. I didn't spend much on the switches anyway, so if it doesn't work well, then I'll just find some other way to get input. I'm going to make a 4x4 or 2x2 test build before I construct the whole thing anyway. I've seen the Sparkfun button pads, but those can get expensive quick. Arrogant as I am, I believe that I can find a more economical way to build a Monome-type device.
@Lyght: They can be found with a quick search for "vending machine capsules." Here's a link to a site that supplies them, though I've seen them for cheaper:
(FYI, I haven't got any yet. I'll post an update if/when I find a really cheap place to order them.)