I have an a99, and the 16-50/2.8 (crop lens). When I use them together, the a99 automatically goes into crop mode. The EVF and LCD "zoom in" (in effect) and you see what you would on an APS camera. The AF points are enlarged and spread out more (technically they are in the same place but relative to the crop it appears more spread out.). There is no option to turn off the crop mode, at least with that lens. Overall, it works exactly as you would expect, the camera simply changes itself into an APS camera. The file size of the RAWs or JPGs will be smaller, since the megapixel count is less. You buffer can hold more of the smaller files, if you are doing a lot of bursty shooting.
I have also switched to crop mode with full frame lenses attached, this also works as expected, the a99 turns into a 10.3Mp APS camera. With a FF lens you can turn the crop mode on and off.
When I put the Samyang 8mm fisheye on (which is marketed as an APS lens) I can choose to crop or not, since the lens in unchipped and the camera does not know it is a crop lens. With this lens, I get a slightly chopped circle, which is quite usable (more than "quite," I use it this way all the time!). My best guess is non-Sony lenses will give you the option, and only Sony lenses in the camera's internal DB will do the mandatory auto-crop. My experience with the Samyang leads me to believe that many crop lenses have useful content outside their stated use, but of course every lens will vary. Some might show soft and dark corners but otherwise be useful, some might clip pretty hard just outside the APS range.
