DD started full-day, 5-day-a-week preschool at 3. There were some separation / transition issues for the first couple of months (which mostly went away when she moved from the 2s-and-younger-3s class to the older-3s-to-5s class), but she'd been desperate to go to preschool, and really enjoyed it.

She would not have enjoyed preschool at 2, so we kept her home. And she still turned out pretty well, despite having been the center of attention until 3.