For at least the last 4 years, we've had to put them on all bedroom circuits here in Orange County, CA. This usually requires 2 to 4 AFCI's per panel.
As Reno stated, I have also had problems with the lighting circuits, and dimmers seem to be the common thread when those problems arise. I'm not an engineer, but it seems to me that perhaps chopping the wave form, as most dimmers do, creates a "bug" for the AFCI system that causes problematic tripping.
I also got a call about a month after completion of a new construction home related to a constantly tripping AFCI breaker. I performed every test imaginable, including a megger, on the home-run portion of the circuit, and each branch (while isolated), and was unable to find a problem. A new AFCI lasted about another month before it started doing the same thing. I had to finally conclude that something the resident was plugging in was causing the problem, and just replaced the unit with a standard breaker. Problem solved, although I'll admit to cheating it.
Conclusion? AFCIs are not completely ready for the '08 NEC requirement. There are technical issues that the manufacturers need to iron out and, IMHO, the NEC jumped the gun on this one.
Also, there's the issue of panel space. They have yet to develop/release a tandum AFCI.
