Detailed hands-on preview on http://www.dpreview.com/previews/SonyDSLRA900/
This also has a full review (the best I've read so far on the web):http://www.imaging-resource.com/PRODS/AA900/AA900A.HTM
With some comparisons with Canon's flagship - the 1DSmark3. Interestingly the A900 holds it's own in IQ, with the Canon being marginally better. Obviously this is 3x the price of the A900, but also now 3 years old and due for replacement.
I think the most interesting comparision will come when the Canon 5Dmark2 arrives, since this will be another 'semi-pro / prosumer' high resolution full frame camera at a similar price (supposed to be 21MP)
For me - the lack of pop-up flash makes the camera a non starter. I'd like to think I could live without it, but my camera serves many purposes and it's not as if the A900 is some High ISO Monster that you can shoot without flash.
IQ looks very simular to the A700 at 100% - which was too be expected and a big achievement for 25MP, but I don't think I need 25MP. I would assume long exposures would also be similar to the A700 - just with bigger files. I'd like the bigger and brighter viewfinder, but not sure I'm willing to pay £1800.