According to:

"Sony’s new Alpha 850 will be identical to the Alpha 900 in size, handling and external design – including the omission of onboard flash. The camera, expected to be launched before September, uses the same 24.6 megapixel CMOS sensor as the Alpha 900 but has – it is rumoured – only a single BIONZ processor, and a very slightly modified viewfinder. It is shipped without the Remote Commander (this becomes an optional extra) indicating that minimum retail price is Sony’s aim.

... The Alpha 900 uses twin BIONZ processors to achieve 5fps. It also uses, as far as is known, Apical’s proprietary IRIDIX processor to handle the DRO, DRO+ Auto and Manual image optimisation. ... The Alpha 850, however, achieves 3fps using a single BIONZ or so it is rumoured* – and it does have DRO, and DRO+. ...

The second difference between A900 and A850 is that the optical prism shows 98% of the frame, not 100%, but does so at exactly the same 0.74X magnification and with the same 20mm eyepoint. ...."