I did some poking around and went with Deborah's suggestion to cast on about 10% fewer stitches than I thought I needed based on my measurements. This seems to have worked out fine with the experimental sock-in-progress, though exact gauge is way less important with socks as stretchy as these.
Incidentally, in poking around, I found misocrafty's super-helpful sock knitting resources. I'm pretty sure there's an answer there for every sock question anyone might ever have.
Later on I saw (blogless?) Bettina's comment about how sometimes the sock yarn wants a size 1 needle and sometimes it wants a size 2. This "listening to the yarn" thing is a wonderful idea. Normally, I think I've been assuming that all yarn wants to be knit crushingly tightly on whatever size needles might sort of get me gauge, regardless of whether this is what would best serve the yarn. Because the yarn is not the boss of me.
But if I swatch, I have the opportunity to adjust for my tight gauge and get a soft, drapey fabric (if that's what the yarn wants) or a tighter fabric (again, if that's what the yarn wants).
That way I can knit something that fits AND show off the yarn to the best of its abilities. It's not wasteful or boring to swatch. It's respecting the yarn that I love so much. Awesome.