When to plant tomatoes

From http://www.helpfulgardener.com/vegetable/2003/tomatoes.html:

"It is OK to plant tomatoes in the garden when the temperature is a consistent 50° F, but plants won't begin to set fruit until the overnight low is regularly above 55°."

I tip I've always used is that when the Apple Tree is in Full Bloom, you can plant tomatoes. that is because the Apple Tree will not bloom until the soil is about 55 degrees.