It always happens. You cannot sell a better camera for less, if you want to keep selling your more expensive model. And why would a 20D/30D ownner want to buy a camera that is more faff to use and as said above SD cards are cheap.
If you want to see a fantastic example of removing features/usability to target each price point, look at Polar Heart Monitors.
The interesting thing that appears to be happening here is that the cheaper 450D really does offer some useful improvements over the no doubt better-built 20D, 30D and even 40D.
Those who occasionally drop their camera or who have had the experience of getting caught in a downpour with their camera which subsequently refuses to work, will probably be willing to forgo the improvements in the 450D for the sake of the more robust build and better waterproofing of the 40D, which of course is part of the additional cost.
I'll just go through again the main features I've noticed in the 450D which the 40D doesn't have.
(1) An extra 2 megapixels. By itself, not particularly significant but would be noticeable when upgrading from a 30D. A 50% increase in pixel count is definitely worthwhile, especially when using the full advantage of the cropped format to extend the reach of one's longest telephoto lens.
For example, when comparing an 8MP image from my 20D using a 400mm lens, with a 4.8MP image from my 5D using the same lens (cropping the 5D image to the same FoV as the 20D image), there's a noticeable improvement in detail in the 20D shot. One would not really expect a further noticeable improvement when using a 40D in place of a 20D in these circumstances, except perhaps at 400% enlargement on the monitor.
However, I'm fairly confident the 450D would
provide a further improvement in these circumstances, compared with the 20D.
(2) +/- 3 stops EC. There are extremes of dynamic range which cannot be captured with just the usual +/- 2 stops EC with exposure bracketing for the purpose of merging to HDR. Such scenes would include the interior of one's hotel room with a view of snow-capped Himalayan peaks through the window.
Of course, you can get around this limitation by using a tripod. However, CS3's auto-alignment of images is so good, you can now merge hand-held shots, although sharpness is going to be limited by the sharpness of the longest exposure.
(3) Auto-focussing in LiveView mode. The 450D has it. The 40D doesn't. The 40D's autofocussing is phase detection which is expensive to implement at maximum apertures of f8, so the 40D doesn't autofocus at all with 400/f5.6 lenses plus 1.4x extender. The 450D which employs a contrast detection system probably does autofocus at f8 and/or in LiveView mode, although this remains to be investigated.
Setting aside ergonomic issues and robustness, the main feature advantage of the 40D is it's 6.5 frames per second as opposed to the 450D's 3.5 frames per second.