B.R.
Is she starting preschool because you have to work, or just for the experience? If it's just for the experience, I think I'd consider finding other outlets to get her around children and wait for preschool until she's mastered the potty. Of course some preschool teachers are great about helping potty train too. If you are fortunate enough to have one of those, it may actually help her to go to preschool.
One thing I've found in my years of working at various preschools is that too often the rules of the center are not designed to take into account the different developmental patterns of children. A lot of them insist on a child being potty trained earlier than some children are ready for it, or hold them back in a younger class until they have mastered the potty, because they don't have the older children's classes set up to accomodate training. I understand the dilema the centers face because they are so busy with teaching other skills once the children are three that there isn't much time to be taking one or two children to the potty. But, if you have the option of avoiding that sort of conflict for your daughter, I certainly recommend it.
I know you said she's 2 1/2, and am sure you are anticipating her to be totally trained by age 3. My experience says that isn't always the case, and even though it would make life easier for us adults, it isn't always the best thing for the child to be pressured to learn earlier.
We are dealing with a granddaughter who will be three this month and is still not totally trained. When she was two, we thought she'd be finished training before she hit the 2 1/2 year old mark, but sometimes they start out great and regress for a while. Her sister did the same, but when she finally got the hang of it, it seemed like she was trained overnight.
When I raised my children, I was one of those moms who tried to train early. Now that I'm a grandma, and have worked with so many children in preschool and daycare, I'm beginning to think we'd be better off with most children to wait until they are your daughter's age to even begin introducing them to the potty.