Move them from the heating Pad immediately. Otherwise you will have a leggy thin weak plants. I don't have this problem with Tomatoes. I have it with Cantaloupe.

I had this happen with cantaloupe too, but only one seed. I had two seeds in the same pot, (so exactly the same conditions). One had the cotyledons 1" above the soil the other almost 2 1/2" - weird! I just deep planted it when I transplanted, seems fine so far.

Yep, I did pull them off the heat. Good point though, I'm sure they'd be even taller then. Yikes. It's driving me nuts! I checked the soil temp - 72F (no heating pad, under lights in a 70F room)

I won't be home until late tonight, hopefully they didn't keep on growing taller.

hmmm, I have the same problem, but with a twist. I started my seeds using the wet paper towel method on heat. As soon as germination occured I planted them and put them under light. Within two days they were hitting the lights. On another occasion I simply planted the seeds directly and put them under light immediately. "Finally" they sprouted and grew at a normal rate. I think the extra heat sends some sort of a message to them to grow quickly. At any rate, simply transplanting them deeply seems to solve the problem in the long run.