He might have trouble getting a straight answer from Stihl. They sold a few here and then quit selling them altogether. I deal with two very good and knowledgeable saw shops, real saw shops, and they're scratching their heads too. The answers as to why are all over the board, everything from needing to sell the last of the 660s or unspecified problems with the AV to you name it, which tells me that Stihl isn't giving out a lot of information to their dealers either.
All I know, and all I can say, is that the 661 got the crap ran out of it in field tests by loggers for quite a while. It was run hard, abused even, with the purpose of looking for weak spots in the design. It looked like a good saw from the start and I can't imagine any late appearing defect serious enough to cause a recall.
If I had to believe anything, and I'm not saying it's the actual cause, I'd say it's probably a lot of unsold 660 inventory that's causing the hangup. Our local prices on 661s aren't much different from the 660s but I think that's about to change.