Hi vahonen (or Samuli?),
Sorry to hear about the theft of your stuff.
I am also interested in the differences between the A850 and A900, but it's very hard to find solid information on this. In fact, it's very hard to find any information on the Sony cameras and lenses! The various Sony websites are fragmented and useless (at least the English ones; the Japanese ones might be more useful). Nikon and Canon are vastly better at marketing their cameras to photographers. They understand the importance of websites devoted to the details of the cameras, PDF brochures with detailed information, "white papers", etc. Sony does not, or doesn't care.
I did manage to pick up a printed brochure of the A850 and A900 (one brochure for both) at a trade show in Paris. It's in French and doesn't seem to mention any differences that aren't in the specifications. However, it's not very detailed so it's hard to know if it's comprehensive. I have not yet found this brochure on the web.
I did find a brochure on the A900 (though not the A850) via Google's "filetype:pdf" search function. I could not find this on any Sony website directly, even though I looked, which shows how bad Sony's websites are. It's in English and may be of interest to you:
I think the viewfinder differences between the A850 and A900 are restricted to the coverage. Certainly they use the same focusing screens. And yes, there is an optional focusing screen to improve manual focusing. It's called the Type M: model number FDA-FM1AM. There's also a Type L focusing screen (FDA-FL1AM) which has grid-lines. Sony say the following about the screens:
"According to your preference or shooting needs, the standard Type G spherical acute matte focusing screen that comes with the A900 can be replaced with an optional Type L spherical acute matte screen or Type M super spherical acute matte screen. The L type features grid lines to enable easier framing, while the Type M is particularly well-suited to bright lenses with an aperture of F2.8 or greater, and features a high-dispersion screen that makes it easier to fine-tune the focus manually."
Regarding other differences, I found the following YouTube video of an interview with (I presume) a Sony marketing guy:
The video isn't very interesting, but at 1:51 the interviewee says this:
"...the frame-rate on the Alpha 900 is 5 frames per second; on [the A850] it's 3 frames per second. Now, that has been for us a considerable cost-saving, because the mechanical relationship is different between the shutter and the mirror-box assembly."
Of course we all know that marketing people often don't know their product and/or lie about it. So I don't know what this means. If the difference exists, it could just mean that Sony managed to find a way to save money without harming performance. It might even mean that they improved the mechanics based on experience, but they're obviously not going to tell us that the A850 is better than the A900. It could alternatively mean that the A850 has poorer mirror damping than the A900, in order to save costs. We simply don't know. My own guess is that there is no difference and the Sony guy is simply misinformed.
I suspect that there is less cost difference between the A850 and A900 than the price difference would suggest, i.e. the A850 is simply a way to reduce prices without letting market forces drive the A900 price down. Sony is generally averse to letting prices rise and fall with demand, and they may want to keep a higher price point for the A900 replacement.
The A900 is unusually cheap in the UK because the value of the pound fell between the introduction of the A900 and the A850. The Nikon and Canon replacements for the D700 and 5D Mark II will cost a lot more than the A850 when they arrive in the UK.
With regard to your question about lens details, it's quite incredible how little information is available from Sony. This is perhaps the best resource that I have found:
It's very limited and the few MTF curves that are shown are obviously not achieved in real life (e.g. they claim 95% contrast transfer at 40 lp/mm for some lenses, which is obviously impossible). There is no information on distortion. The guide is so old that the two Zeiss zooms aren't even mentioned. This is nothing remotely like Canon's "EF Lens Work III". Even Nikon doesn't have anything like that.