Did you start hardening them off? That happens to some of my starts when I harden them too fast and the first tender leaves get a bit windburned and crispy.
Well, whatever it was, it stopped. I very unscientifically tried all of the suggestions, and my tomatoes are looking great! I did the peroxide solution, fertilized & moved them all outside (taking advantage of the weirdly warm temperatures). So thanks for all of the advice!