Tomatoes will not set fruit if the temperature falls below 55. Since our nights are still cool, our tomatoes get their start in "walls of water". The tubes of water absorb the energy from the sun and modulate cool evening temperatures. Later in the season we will remove the walls of water.
I have two more early varieties of tomato to plant. They are terribly tiny, though. I'll let them 'harden off' for a day or two before putting them in the garden with their brethren.