Earbuds that 100% work, for $20
Finally found a solid set of earbuds, with microphone and pause/FF/Rew clicks, for a reasonable price. From Radio Shack, or, "The Shack" as absolutely nobody calls it. They're called Auvio Pearl Buds w/ Mic, and they play with the Lumia900 perfectly.
$20 seems pretty reasonable, for complete functionality, in an accessory that, realistically, I'm going to break or lose in the next few months. Other commenters have mentioned the problem with the Purity earbuds that they won't work their magic when the screen's gone to sleep - these seem to work immediately every time.
Full audiophile disclosure: I can totally tell the difference between $400 Beats and cheap earphones, I just don't really care. Somebody else will have to give a review of the fidelity. These are way better than I need. They come in several saucy colors, too.
edit: I found something that doesn't work, and it's really basic - click-to-answer-an-incoming-call doesn't seem to work. I hadn't received an incoming call when I posted my initial review, now I'm oh-for-five. Everything else works, all the weird stuff listening to music and books is still aces.