A* (A star) grade is only applicable to candidates starting their course in September 2008, and only concerns to candidates taking A2 levels in May/June 2010 Session. A* doesn't apply on AS, neither on A2 alone, it is a combined overall grade for the whole A levels. But CIE didn't disclose the mark boundaries for getting an A* grade. Nevertheless, it is going to be implemented in May/June 2010 session.
For further information, please refer to THE STUDENT ROOM forum. There is a lot of useful information there.