Dear V.,
My son, who is now 4, has had a lot of the same issues. He's been in some kind of preschool (very part time at first), since he was 2, but until 3-and-a-half, he really hung back and clung to the teachers.
And then, we set up a playdate with just the right kid.
I know this sounds like a simplistic solution, and I honestly didn't believe it'd work until we tried it. But I sat down with N's teachers and asked them to help me identify a little boy who was calm, patient, and gentle. By some miracle, there was a little boy in his class with all those attributes, plus some common interests. I contacted his mom and explained that N. was sweet and bright but very, very, shy -- and would she be willing to do a playdate? She was, and we got together in neutral territory (playground). It took a little stage-managing for the first half-hour, but after that, they were playing together beautifully. And that was it. These little boys are now best friends (though it's a bit of a leader-sidekick friendship, with my guy as the definite sidekick). As soon as this friendship took off, N started loving school. Can't wait to go in the morning, doesn't want to leave when I pick him up. I'm now trying to get him to expand his circle of friends beyond a grand total of one, but the difference a playdate made is unbelievable.
So, that's what I recommend. Sit down with the teachers, ask them to play matchmaker, and try to take it from there. I also think super-shy children do better in preschools where there are more structured activities. If your daughter's in a school where children are expected to spend the better part of the day socializing, that just might not be a good fit. Finally, it helped a lot for me to really sit down with the teachers and explain that my son was very verbal and (at home) very assertive. When kids withdraw, they don't take risks, and then they start functioning at a developmentally earlier level. With my son, two teachers actually suspected a developmental delay -- and this is kid who can read 50+ words and do addition and subtraction at age 4! When I explained that there was a bright, verbal kid hiding deep within that silent shell, they worked much harder to bring him out.
Sorry that was long, and good luck.
Mira