Canon's recent introduction of the 18mp cropped sensor 7D has generated a lot of snarky comments from one side and many wrong comments from the other. While I too would rather have a solid 12mp cropped sensor body than an 18mp one, it's not so much noise that I'm worried about. We still have a ways to go before we top off on noise handling with sensors, and 4 micron photosites with continuous microlenses should be perfectly okay, noisewise, at least at the ISO values I tend to use.
Long ago I calculated that DX (and Canon's APS-C) would top off at somewhere just above 20mp, certainly by 24mp. The reason is that as you push pixel increases you run into a number of other factors where you are either getting to the flat part of the increased performance curve or actually over the peak. Diffraction is certainly an issue with so many pixels, and this interplays with lens quality. The actual gain in true resolution once you get beyond about 14mp starts to flatten below the curve we were on from 6mp to 12mp and eventually nears zero as you get above 20mp. Note that Canon has introduced new EF-S lenses with the 7D. I suspect that they might have had to. I'm not sure the old mid-range EF-S zooms would be fully up to 18mp with increased diffraction stealing a bit of MTF (i.e. the old lenses would show less than the expected resolution increase from pixels alone).
Another comment I've seen is "more pixels are great for cropping." I'd rather have fewer pixels with the right lens than more pixels and crop. Moreover, those that trumpet the "just crop it" mantra don't understand perspective, a key element in photography. Ironically, such statements make an 18mp camera more of a beginner's camera than a prosumer's. Still, there will be a few wildlife photographers who value the extra pixels, at least if they're using quality lenses wide open.
Back in 2003 I wrote that I believed that the cropped sensor cameras would probably top out at around 24mp. I still believe that. For cropped sensor cameras a 12mp non-Bayer (three-layer) camera would outperform a 24mp Bayer camera, so I suspect that we'll see a reset just about the time we get to the limit. Then again, we've seen electronics go past meaningful limits before, mostly because bigger numbers are easy to market ("it's got more!").