rbarryyoung, I don't quite follow your code, there. Somethng missing?
Anyway, it did inspire what may be the most obscure solution yet. We define an integer to hold what is essentially an array of bit-switches, then test the birth month against it with a modulo-remainder.
Declare @SelectionSwitches int
Set @SelectionSwitches = power(2,2)
Declare @Dob datetime
Select @Dob as Dob
,case when ((@SelectionSwitches/power(2,datepart(mm,@Dob)))%2)
then 'Yes' else 'No'