When should DD enter pre-school or montessorri school?

[cmkeough]

DD is 9 months old today and as I was driving I passed a preschool and it hit me...oh, crap when do (or do we) need to go to pre-school or montessorri? I live in N. Arlington and really don't have a clue as to when/what schools would be well suited for development of DD. Currently we participate in a nanny share M-F and although it's great I do want DD to have more of a social set too.

Any suggestions would be greatly appreciated.
Christina