As I said before I have had long talks with Harvey Johnson from C-H about these AFCIs. The real problem is a series arc that would burn your house down if it occured in the pile of dust bunnies under a bed from a pinched zip cord might look a lot like the normal arc you get in a hair dryer "pulse heat" switch if you are slow on the trigger.
The circuits are really trying to find a sweet spot between a "safe" arc and an "unsafe" arc without knowing where the arc is. You can see some nasty arcs in your garden variety snap switch if it is dark in the room where the switch is. Try it sometime. Use an extension cord on a switched receptacle and put the load in another room, then turn the light off and flip that switch a few times. It makes you happy that thing is in a box. That's a safe arc. Imagine the same arc in some cotton fuzz and you see the problem.
Parallel arcs are easier to detect because you are just looking for verey high current spikes but you also need the sweet spot between the normal LRC of a big motor like a vacuum cleaner (brushes etc) and that pinched zip cord under the bed.
This would be a lot easier if NFPA allowed us to put the protection closer to the load.
Then you might be able to dial in more protection where you should not expect "normal" arcing.
I am guessing this will be tossed around a lot between now and 2008 or 2009 when most places will actually adopt the code
