Don't get fooled by this flawed test though. For a little more accurate idea bout noise performance read the dpreview reviews of these cameras (and see the samples given there).
Originally Posted by sparkerjc
The Nikons all do chroma noise filtering, and they lose details as a result.
You can not fully turn that off, even in RAW the Nikons filter some noise.
The Olympus uses a HUGE amount of filtering, losing even more detail in the process.
If you filter that noise in the photos taken with a Canon and Pentax, things level. The 40D has the edge over all these cameras, the XTi does not do bad either.
The Pentax K10D, Nikon D200/D80/D40X and Sony A100/200 all use the same sensor family. The Nikon D40X uses the most processing of all three Nikon models, which you can see in high ISO photos (lots of detail being eaten away, strange resulting artifacts).
The best advice for high ISO shooting: make sure you expose well.
Canon EOS 350D, Tamron SP AF 90mm F/2.8 macro, Sigma 18-50mm F2.8 DC EX, Canon EF 70-200mm f/4 L USM, Tokina AT-X124 Pro 12-24mm F4, Soligor 1.7x C/D4 DG Teleconvertor, Manfrotto 724B tripod, Canon Powershot S30