I have to loudly disagree here!
Yesterday, I was at Fry's Electronics and I saw a playback of a Blu-ray movie on a 20" LG Electronics W2053TQ 1600x900 LCD display fed from an HP Pavilion Elite d5200t with an ATI Radeon HD 4650 graphics card, with playback using PowerDVD 9 software. It was very sharp with excellent color details, to say the least; when the salesperson switched to a regular DVD the loss in detail is VERY visible, especially with a big loss in sharpness. If you can see the difference between DVD and Blu-ray clearly on a 20" widecreen monitor, you can see it even more vividly on a 24" monitor used by the higher-end iMacs. Remember, because you are working at a desktop computer you are sitting a LOT closer to the screen than a regular home theater setup, and as such even on a 20" monitor you can tell the superior sharpness of a Blu-ray disc.
The perceived differences are a subjective issue. You & some people can see a clear difference on even smaller screens, many others can't. Fact! It's been proven times over with double-blind trials showing that on screens much smaller than about 50", most people can't tell the difference between proper HD 1080p TVs showing Blu-ray, & relatively cheap, 1080i (interlaced) TVs, showing standard DVDs. AFAIC, unless you can afford & have room for a large HD TV, standard DVDs on 1080i TVs seem to provide a good enough viewing experience for many people.
FWIW, I don't want Blu-ray putting iMac prices up even further, but I think Apple should offer it only in the more expensive iMacs for those who want it, leaving it out of the cheaper iMacs for those who don't give a fig for it.