My daughter WAS, and I say it in past tense because I withdrew her from the program, enrolled in a community day care center since June 'til early August. About 2nd week of July, we tried out the Galileo program for her as well. The day care session lasts for about 1.5 hours while the Galileo session lasts for an hour. Since public day care center sya, there were about 25 students - and they were in a small classroom lang. As in, small lang talaga. I really just wanted to try out the classroom environment for my little girl (without the need to spend too much), kasi naging successful yung summer schooling niya... Unfortunately, the environment at the day care center wasn't the best fit for her. Too many students cramped in a small space, hindi niya masyado gusto. And well, I personally didn't like the way the teacher handled the class.

Saka ayaw talaga pumasok ni baby pag doon papunta. As in nandun na kami, ayaw pa niya pumasok. In contrast, she super enjoys naman her Galileo sessions. Twice a week lang sya, pero ang dami na niya natutunan in 6 sessions pa lang. And talagang very enthusiastic sya. So we decided na stop na lang muna sya sa daycare and continue with Galileo - we don't want her traumatized by the idea of schooling.
I couldn't say naman na she's not ready for the classroom environment, kasi during her summer session naman, we never had a problem at all, from going to school, leaving her there, classmate interaction, etc. She's also a fast learner and takes instructions easily. Yung environment lang din siguro ang hindi naging good for her sa daycare.
Next year she'll be four na by start of the school year, and since continue naman sya with Galileo, and summer school ulit next year, I'm pretty sure na she'll be much better adjusted na. We already found an awesome progressive school for her, pero we're also looking into a traditional school. We'll see next year.
So I think, as far as readiness of the child is concerned and how early to start schooling... you have to really let the child try it out for you to find out.

But one thing's for sure, any younger than three may not be ideal for a CLASSROOM-environment type of school/sessions. Even at three touch and go pa din depende sa bata at sa kind of school na papasukan niya.
If you discover na di pa talaga ready, well, let our kids be kids muna. They won't be like that forever.