Northlight Images EOS-1D MkIV rumours page
has some speculation about the future for the 1D line including, trivially, the possibility of a new name following the AF debacle. More interesting, in my view, is the possibility that Canon may drop the APS-H (1.3 crop) sensor in favour of a full-frame sensor, this in response to the Nikon D3.
That leaves open the possibility that Canon can use an APS-H sized sensor in a future semi-pro model without hurting sales of their "pro" models, although the position of the EOS 5D replacement (5DMkII) might be slightly less clear. The benefit of better image quality that the larger photo-sites allow is a given but it seems to me that this move could only happen if Canon could arrange physical compatibility with their EF-S lenses. If that means using the central part of the sensor because of vignetting or other optical issues Nikon have, of course, opened that particular marketing door with the D3.
So, will we have an EOS 40D replacement (EOS 50D or even 60D) with an APS-H sensor? Such a camera would be a cracking answer to Nikon's D300 with its 1.5 crop and has a beautiful fit with some of Canon's finest glass. Instead of a full-frame equivalent of 27-64mm on an APS-C (1.6 crop) sensor the EF 17-40mm f4L USM equates to 22-52mm giving significant wide-angle coverage. Similarly, instead of the 38-168mm equivalent of the EF 24-105mm f4L IS USM we get 31-136mm which is arguably the perfect walk-around lens. I could continue but you get my drift. The only losers would be those who rely on the longer telephoto lenses where new glass or, compatibility allowing, Canon Extenders might have to be purchased.
All speculation, of course, but if the demand is there I'm sure Canon could oblige.