Friday afternoon I got on the bike and went around the block a few times, and my back didn't hurt.
Saturday morning it still didn't hurt--at least, not more than it normally hurts when I get up in the morning; something about lying in one position all night doesn't agree with it. (Fear not; this is going away, slowly, as my Soft Tissue Injury™ enters the final third of its healing process.) But the point is that being on the bike and going around the block didn't aggravate anything.
So this morning I biked in. And discovered two things: I really missed the biking, and I am woefully out of shape. While this was not quite the overheated shaky-kneed don't-turn-your-head-lest-you-puke experience of my first few bike trips, I'm also not exactly in top form. My legs are OK, which implies I built up more muscle than I thought, but I just couldn't maintain speed. And I've forgotten how to efficiently switch gears and will have to learn it all over again. Being on birth control these days didn't help, since I'm getting low-level pseudo-pregnancy morning sickness from the hormones, but fortunately the pop machine in the basement of my building has ginger ale.
It remains to be seen whether I'll be able to bike tomorrow. I am reasonably certain I'll get home OK, though I won't guarantee I won't be walking the bike up the hill at the Penn-Butler split, but a night's sleep could do interesting things to my muscles