i just repeated the test with the 450D ... same settings. Im getting lower values now ... god knows why:
iso100: 9.7
iso200: 10
iso400: 11.2
iso800: 14.5
iso1600: 21.6
That's still a bit high, though. Even with the extra pixels, that gives the lowest DR of any Canon DSLR from the 10D on, at ISO 100. Looks a little better perhaps than the 40D at ISO 1600, though. This is in terms of DR; in terms of absolute signal-to-readnoise, I can not comment, because the exact improvement in quantum efficiency in the 450D is unknown.
This is an interesting pattern; something similar happened with the Pentax K10D -> K20D upgrade. The K10D had the lowest read noise of any DSLR at base ISO, but it's highest ISO was pretty much the same thing as base ISO pushed. The K20D was interesting me because it was going to be 14MP, and I had hoped that the same low-ISO read technology might be there to give the K20D tremendous DR at base ISo, but it turned out to have one of the worst read noises of any DSLR at base ISO.
The pattern that seems to be emerging is that it is technically difficult to design a single camera to have good high-ISO noise and good low-ISO noise with higher pixel densities. The reason may be as simple as unoptimized, fast readout that punishes the low ISOs more.
If this is the trend of the future, I really hope that manufacturers will start making different bodies for different ends of the ISO spectrum. Implementing compromised high-ISO in a low-ISO body is easy, and all P&S cameras and most existing DSLRs already work that way. Implementing low ISOs in high-ISO bodies is more problematic, because you need neutral density effects for long shutter speeds.
Rest assured, however, that the higher low-ISO noise is not directly due to pixel density per se; there are P&S cameras with 2 micron pixels that have similar read noise. IMO, some kind of compromise issue is involved.
If Canon (or any manufacturer) could make a 100MP APS sensor filled with 2 micron pixels, with uncompromised read speed, while maintaining base ISO read noise at less than 3 12-bit ADUs, the camera could perform as well a current DSLRs at high ISOs, and be much, much better at low ISOs than any current DSLR. That's a long way from this year's technology, though, and with the coarser pixel pitches used in DSLRs, they still need to have designs optimized for high ISOs, for good low-light performance.