Wow, you guys are good. After Kelly posted about our potential tomato and squash problems, you all responded with a million helpful and interesting suggestions—we even got the advice of a plant pathologist and a biologist (thanks, Taylor and Miriam!). Alan even put up a post about similar problems in his garden, along with photos that illustrate the curly leaf problem better than ours did.
After reading all your comments, and based on the weather so far this year and the general condition of the yard and garden, we're rooting for either a water issue or a magnesium deficiency. Whichever. Maybe both.
Lots of you suggested that the curly tomato leaves could be the result of irregular water, which makes sense because as I'm typing this the tomatoes are getting rained on for only the third time this year. Our rain barrels haven't been able to keep up with the drought, and so the garden has been running on minimal water. Vegmonkey probably said it best: "the tomatoes look like they need a damn good watering." Bloody right, VM.
The magnesium issue that many of you mentioned also seems about right—aside from eggshells and compost, we didn't add any amendments to the beds this year. The yellowing leaves seem to match with the photos we've found of magnesium-deficient plants, and even the basil (around the tomatoes) and the nasturtiums (around the squash) have some splotchy yellow leaves, which is definitely not typical for them in our garden.
Of course, we could still have an unfortunate disease—both of the consulted scientists mentioned mosaic virus, among others—but since there's nothing we can really do about that we decided to cross our fingers, buy some epsom salt, and give everything a damn good watering.
We didn't even know where to find epsom salt, so we wandered around the store for a while until we found this surprisingly large and cheap carton of the stuff:
And we watered it in a bit:
We watered just enough to get all of the salt dissolved into the ground because a big storm was rolling in as we were outside. Tomorrow we'll give all the plants some fresh compost and prune some more, and hopefully within the next week or so we'll see some signs of recovery. We'll keep you posted.