I find it very hard to add additional information to the Mac OS X Address book : there are only some basic fields, but when you want to add something like a birthday field I can’t find it, unless it’s a custom field.
I know the birthday field exists, as it is added in some of my contacts (don’t ask me how, probably via a long-ago sync with my windows mobile address book).
But just now I found a way to add them quickly – just use your iPhone to edit your contact, select from the bottom of the list the ‘additional field’ and choose the ‘birthday’ field. Easy as pie, and it’ll sync to your Mac Address Book as well !
Now why couldn’t this list of all the additional fields be just as easy and available on the mac ?