Whilst this is a very old question, it is so important in getting a date of birth input correct, especially in a registration form.
I think no one has done this better than the google accounts sign up:
Select the month first from a select box and manually type in the date and year. Simple.
Easy to validate. Easy for users to get right. No scrolling back the years in a select box from 1900-the present year. No need to update a 'day' select box based on month input. Works great on web. Works great on mobile. Works for all locales eg 01/10/2014 - is that the 1st October or 10th Jan? I expect we'll be seeing this birthday picker format a lot more in future.
A datepicker is just a poor solution all round. So many clicks! In my opinion, a datepicker is only useful when knowing the day of the week is important eg booking tickets.
Update 2/6/2016: I am from UK, so I am more familiar with day/month/year formats, rather than month/day/year. However, users who will use their cursor to select the month, I believe it is much easier having the select box first, rather than going input > select box > input. The comment date is 2nd June, not 6th Feb ;)
