I have tomato seedlings in my cold frame, but I will not plant them before May 1st -- and even then I tend to wait until mother's day. I'm keeping on my normal schedule. You never know if a cold snap is around the corner.

The most critical thing is the soil temperature. If thr soil is not warm enough bean seeds will rot and tomato plants will be very unhappy and prone to insect attack(flea beatles)

University of MD has a great web site. There is a planting guide there that is really good.
Here is the web site: http://growit.umd.edu/