Not too early to start, although it seems like nanny shares tend to fall into place only a few months (or less!) in advance. I called many in home daycares 6 mos before the due date, and almost all of them told me to call back a few months before I needed care to start and they'd let me know if they had spots at that time. But you could visit them now, and have a priority list ready to go . . .
And for the big daycares downtown, 6 mos might actually be too late . . . all the ones I talked to that were even adding to their waitlist told me that a 9-12 wait for an infant spot is normal. A couple told me the waitlist was already full, and one said, "by the time you get off the waitlist, your child will no longer be an infant!"