Having now had my A900 since October 2008 (over a year now), I have not had even a whisper of a problem. Have carted it along across the US, to Europe, South Asia (extremely humid/wet conditions) et al and the camera has never misbehaved in any way, shape or form. The lenses too have been exemplary. I would say that this theme is pretty much valid across the hundreds of Sony A900 users on the dyxum forum (and also the several on this forum).
If I had it to over again, I would have bought an A900 last January. Nonetheless, I'm liking the A850 very much, except for this one dust/moisture issue. If Sony resolves it without any issues, then I will be singing their praises. I've heard that Sony is very good about service; unfortunately, my experiences with Nikon service are on the negative side (which was one more reason for me to adopt the Sony system for my high resolution DSLR).
My copy of the CZ24-70/2.8 is also headed to Texas. From 24mm to about 28mm the extreme corners are not good at all (no matter how much I stop the lens down). Again, I'm maybe a little unlucky with quality control, although the lens issue is less unheard of than the camera issue (I've never heard of the problem I experienced with the camera ever happening to anyone before).
Wonder if there is some difference in the build quality between the A900 and the A850, even though your case is the first I ever heard, where such a situation has happened with a FF Sony camera.
I wonder that myself, but I think I was just unlucky. Iliah Borg wrote somewhere that he's seeing more variance in the spectral responses of the A850 than he sees in the A900, so I think this is a QC issue. I'm very happy with the image quality I'm getting from the A850, so I may end up getting an A900 next year as I want a back-up and prefer to have two cameras when shooting so I can minimize lens changes.
Hope it gets fixed soon.
Thanks, I'm optimistic right now and expect to have the camera back by the middle of January. Like I said before, I'm refraining from discussing this at length as I want to wait to hear what Sony has to say about this and will be including the service experience as part of the overall assessment my copy of the A850.