We're working on birthday emails now too and have been going through all the threads that discuss the advanced wait step, date work around for birthday card, etc but, we haven't quite nailed down how to do it in our own Instance...yet!
1. We don't use SalesForce so we can't create a special field to write to Marketo for a trigger
2. We can use a datetime option special field however, it seems as though there are similar issues for that field type.
Smart List: Includes any customers w/ a birthday in the past
The resulting schedule looks like this (we've used test accounts and entered a number of birthdays):
This is the qualified leads (there should be about 6 more but they haven't shown up any time we create this campaign)
We're eager to hear of any other options that anyone can think of.