First, our plants were in a greenhouse till you bought them. For best results, they need to be "hardened off" in the 5-10 days before transplanting into your garden. Hardening off is the process of acclimating plants to outdoor conditions of direct sun, wind, and cooler night-time temps. This is accomplished by exposing them to an increasing amount of outdoor conditions until they tolerate at least 6 hours of it. Start by putting them in a sheltered place, and take them inside at night if there is danger of frost.
Second, the minimum soil temperature that tomatoes will tolerate is 50 degrees F; the optimum soil temperature for tomatoes is 70-95 degrees F. They make soil thermometers, but our tax dollars helped developed data available from the excellent California Irrigation Management Information System (CIMIS). Average soil temps recorded during 2009-2010 in several representative counties tells it all:
(Soil temperature readings taken 6" deep)
Remember, tomatoes are from the tropics, so keep them comfy...