Revenant (9/7/2010)
The correct way is to celebrate a Feb 29th birthday on the 28th, that's how the Roman (Catholic) calendar does it. In fact, it is not the 29th that is added on leap years: it is a second Feb 24th and the following four days of February are "bumped up."
The correct way to calculate age is to follow whatever regulations affect your industry or the operational definition of "age" specific to your business goal.
In the U.S., at least, different agencies calculate leap day baby ages differently.