Ps - I really hope Canon resist the temptation to take their 1.6x crop sensor up to 24mp. It'll suffer from softness due to diffraction from f6.0 onwards - mount an f5.6 lens on there and you've got little in the way of options. Even the legendary 300/2.8 II with a 2x TC III will underperform, and leave you with just one aperture option if you want to attempt to utilise all of those megapixels.
Resolution doesn't work like this. In a format, a 24 MP sensor will always out perform an 18 MP sensor whatever aperture you choose. Now even under the best conditions it won't amount to much and may not be immediately obvious in large prints. But it will always be better by some amount. Diffraction is not a hard limit and is not a reason to cap APS-C sensor resolution or to limit your use of apertures.
The current 7D is "diffraction limited" at f/6.9. I regularly use f/8 and f/11, and after post processing even f/16 is fine. More to the point, the 7D is never "worse" than the 10D, 20D, 40D, etc., not even at f/22 or f/32.
High ISO is another non-reason to cap resolution. Noise for an image is determined by technology and total surface area, not pixel size. At least not until you get into the extremely small pixels on some P&S bodies.
The only possible reason to hold back on increasing the resolution is DR. Pixel size is a factor for DR. But if technology eclipses this, my guess is Canon will match the competition. Too many people will see 18 vs. 24 MP as a negative, and Canon's not going to give up that marketing point.
Given the choice I would take 18 MP if it had noticeably higher DR. But that choice may not be what anyone thinks it is, i.e. the DR gain may be less then the resolution gain. And I doubt Canon will give us the choice. Their competitors have settled on 24 MP. You can guess what they will do.