Hi M.,
I agree with the mama, who said you should find a school you like and then call them about their policy.
My daughter's preschool also has a daycare for babies and toddlers. When a toddler is about 2.5, they begin to bring them into the preschool activities, depending upon the child's readiness. After joining the "big kids" for circle time and a few art projects here and there, my daughter was moved over to the preschool room about 4 months from her 3rd birthday.
I personally liked this approach because it wasn't an abrupt change for her - the toddler room one day, the preschool room the next.
It also allowed for the teachers to carefully evaluate each child's readiness before moving them into the more structured preschool environment.
One more thing - many preschools require children to be fully potty-trained. Some are more relaxed about this. When you're calling around, be sure to ask about that as well, so that you have all the facts.
Best of luck!