I am not seeking to defend or attack Adobe so please don't read this as a pro or con on that argument. I would say though that ACR is by no means a mere add-on, it is a highly complex program in itself and the programming then integrates with the programming of Photoshop CS3 in order to provide full (and superb) functionality.
Saying that Adobe should make this fit CS2 so that we 40D users can use an older version might hold water if it was a minor tweak required at their end but that is clearly not the case. Adobe would have to invest serious man-hours into making ACR work with CS2 for 40D and other newer models of dSLR when they gain nothing from it at all.
I believe Photoshop is very over-priced despite being the best photography editing program out there but as a PS user since PS5, I accept that when I buy a program I get the latest stuff available but when the program is significantly updated, I don't expect the makers to add the new functionality to older versions of their software. After all, why stop at the RAW processing? Would CS2 owners be considered reasonable if they also insisted on some of the other new features in CS3 being made available as a free upgrade? I doubt it.
Adobe are stonkingly expensive for their professional level programs but in fairness, they do make a cheaper version available that meets most non-professionals' needs and sell it for about 1/8th of the price of the full PS.
I am certain that if the inclusion of Canon 40D drivers or whatever is needed (I am no software developer so Orangepeel and others will know far more about this sort of thing than me) was a simple tweak that could be downloaded into CS2 to make to work, it would be done but I can't see Adobe investing a lot of time and money into doing so when it gives them n reward at all.
BTW, I can't fly my Concorde add-on very well in Flight Sim 2004 either, had to get Flight Sim X for that
Oh well, my 0.2p worth anyway!
Cheers,
Rob