40D covers what I need and is available today.
Should arrive Tuesday.
Fair enough! But I can't help getting the impression Canon has complicated the decision making process with the introduction of the 450D. It's seems to have some desirable features which the 40D lacks.
I've probably overlooked a few features but the only substantive feature I see in the 40D which could make a difference to the shots you take is the faster continuous frame rate. The 40D would be a better choice for action shots where the precise split second makes or breaks the picture.
On the other hand, the 450D has at least a couple of additional features which could contribute to the taking of a better picture. At least they are features that I would find useful.
(1) 12mp versus 10MP. Not an issue for someone who already has a 10MP DSLR, but for someone coming from a 350D, 20D or 30D, a 50% increase in pixel count has
to be noticeable, using a good lens.
(2) Exposure compensation and autobracketing of +/- 3 stops. This is very useful for HDR shots of really high contrast scenes. A 4 stop difference is sometimes not enough. 6 stops should take care of almost any situation. Try taking a series of 3 shots with a total of 6 stops interval, without a tripod, for merging to HDR, with the 40D. Even with a tripod it's going to be a slower and more cumbersome procedure.
(3) Exposure compensation of +/- 3 stops is also useful for a technique of achieving accurate ETTR using the spot meter mode, which the 450 also has. I learned this technique from Dale Cotton.
Take a spot meter reading of the brightest part of the image in AV mode, fix the exposure with that star button on the back of the camera, then increase exposure by 3 stops using the big wheel, and recompose the scene. Voila! Perfect ETTR!
(4) A big disappointment with the 40D is the fact it doesn't do autofocus at maximum apertures of f8. For all its claims of more accurate and faster focussing, this doesn't translate to better focussing at f8, so it's disabled.
The 450D, however, has (apparently) a very accurate, albeit slow, contrast detection autofocussing in LiveView mode. I imagine this would probably work at f8 and be an advantage for those who use the Canon 100-400 IS lens with 1.4x TC.
The downside of no ISO 3200 is not serious because ISO 3200 is just an in-camera boost which is no different from ISO 1600 underexposed one stop, if shooting RAW.
The incompatibility of memory cards and battery with other Canon DSLRs represents an unfortunate additional expense, though. Probably fine for the P&S crowd who already own a few SD cards, but for many of us the cost of a couple of high capacity SD cards and a spare battery bring the cost of the 450D up to the level of the 40D.