DD1 has had hers done at preschool, though I still don't know much about it. There is less flouride ingested than say, flouridated water or toothpaste. Each treatment is supposed to last 6 months, DD has had it 3 times (she's 3.5 years now) and I don't know if we will do it again. She does have perfect teeth, though.
I assume it is common for state-run preschools. It's offered through our health department and the dentist volunteers to go to the school to do it. Basically, the program saves the parents a huge amount of time and the kids are less stressed because a lot of their friends are doing it as part of "dental week." The dentist also counts their teeth and writes recommendations if something is amiss. At DD's school, everything is done in the classroom with all of the kids taking turns and their regular teachers are there, too. If you opt out of the varnish, you can still allow your kiddo to have his/her teeth "counted."
---feeling like an emu on acid---