I'm reminded from previous dpreview comparisons between the Sony A700, A200 and A350 that high ISO noise is not as good as the equivalent Canons. At ISO 1600 and 3200, if noise in the Sonys is as low as the Canon equivalent, it's usually at the expense of detail, and vice versa. If detail is on a par, then the Sony images are noisier.
This worries me a bit. First impression tend to confirm that this trend is continued in the A900. Nikon has already set the benchmark for low noise from a full frame sensor. It is expected (by me, at least) that the soon-to-be-announced successor to the 5D will match the high-ISO low noise performance of the D3 and D700.
The A700 and the A900 use CMOS sensors, while the others (consumer bodies) use CCD sensors. As we know, the biggest advantage of CMOS is the ability to accomodate NR circuitry into the sensor itself, which is the key advantage that a CMOS has over a CCD, specifically at higher ISOs. A CCD cannot accomodate any on-chip NR circuitry. Thus the CCDs are arguably better than a CMOS at the lowest ISO, but quickly lose steam once the ISOs climb.
But Sony, in addition to the on-chip NR done to the A700 (like all other CMOS manufacturers including Canon does), was also applying NR to the RAW files, AFTER they got generated, unlike the Nikon D300 (which also used the same sensor), which did not apply this "second NR on RAW" in their files. Sony was taken to the cleaners for this (and the A700 sales suffered due to the bad rap from this), by every single site who tested the A700. That story was as of a couple of days back.
But as of YESTERDAY (just before the release of the A900), Sony completely revamped their image processing approach in the A700 (Firmware Ver.4) and using the Nikon playbook, completely removed the NR they were applying on RAW and also made several improvements to the treatment of the images, and the results have essentially proved to be stunning. In one move, they turned the clock back and equalized the image quality with the corresponding Nikon (D300). The dpreview Sony SLR forums are all on fire due to this but the news has not yet gotten to the reviewers.
The A900 uses the same sensor design as the A700/D300 but has slightly larger pixels, due to which the noise is expected to be better than the D300/A700. But the forthcoming detailed tests should reveal how much of an improvement is there.