We put DS into day care (first as an emergency, then one day a week, now two days a week) when he was 10 months old. And honestly, I'll be starting a lot earlier with this next one. I think it was hard on him because he hadn't interacted with a lot of kids before (ie ever!), and he got scared when a lot of kids were around him instantly - just wasn't used to it. So with the next one, I'll be starting at 12 weeks when I have to go back to work. I think it works for our family as DS has problems being independent, and the girls at day care are helping him with that, and his walking skills etc, so I'll be more than happy to have my next one start from an early age.
But its whatever works for your family, and also what YOU are comfortable with.