Apparently the AFCI is designed to trip only if it sees an arc of 75A or more over a period of several cycles, effectively limiting its usefulness to detecting parallel arcs in a 15 or 20A branch circuit.
The ground-fault protection incorporated is set at a level of 30mA, and you'll find many people on this board who are unhappy that the AFCI has been marketed on the basis that a series arc will trip the breaker when it eventually burns through the insulation and becomes a ground fault (or not, as the case may be).
This has really been a hot subject; if you do a subject header search on "AFCI" and another on "arc fault" you'll see plenty of threads.
Just to get you started, here's one of them.
[This message has been edited by pauluk (edited 09-07-2002).]