The a850 spec is now available online in various places. You can DL the manual from various places. (I can e-mail it to you as well, 3.5MB pdf).
In effect, it is nearly identical to the a900.
1. 98% viewfinder (most can live with this). From David Kilpatrick I get that it is the same prism, just masked down a hair. While this seems parsimonious, it apparently eliminates a lot of manual labour in alignment of the 100% mask in the factory while (potentially) resulting in a VF that is not perfectly aligned with the sensor. IOW: expect possible edge alignment under or overflows. Crop critically in the VF to assure that things don't get left out.
2. Single image processor instead of the a900's dual processors.
[UPDATE: Per dpreview, the a850 also has dual processors]
3. Due to 2., above, the maximum frame rate is 3 instead of 5.
[UPDATE: Firmware, not processor limited]
Other than that, it's an a900. All in all I can see buying the a850 as a complement to the a900.
They could have called it an a890, actually...
Price is set at US$2000 in the US (per dpreview).
The other rumour goes that this will be part of a kit with a price drifting up a bit and that the a900 price will be drifted upwards as well. (Rumour 2 / David K.).