Jason, do you use Salesforce?
If so, you could create a checkbox formula field:
"if(month(birthdate_field__c)&day(birthdate_field__c)=month(today())&day(today()),1,0)
Basically, if month/day of birthdate is month/day today, return true. Otherwise, return false.
Then, in Marketo, you'd trip the campaign when this checkbox is true.