I did say 'effectively' medium format...I was thinking more about pixel count rather than full medium format. I know pixel count doesn't in any way define medium format...but if Canon are to produce a Camera that competes for the next three years in the studio...it would seem to me that it would have to be in the 40 to 50 MP range...similar pixel counts to some of the leaf backs and Pentax 645D. But I also realise that this asks the question - are the current optics capable of supporting that sort of resolution? I don't believe that they are.
Could Canon cope with producing a new lens mount alongside the EF? Probably not at the moment. But I cannot believe that Canon are going to give up the prestige of producing a large pixel count camera to their rivals. Both Nikon and Sony are ahead in the raw numbers game...I'm sure Canon will respond in kind.
It has often been rumoured that Canon believe there more recent lenses can handle 40-50 MP and there performance on crop bodies does seem to suggest they've got a good deal more to offer on FF beyond 21 MP.
The real unknown to me seems likely to be whether were talking about one new 5D sized FF body or two, Personally I think that with the 1DX specs/price two has become much more likely since a single body would have to cover alot of ground between amatures moving up to FF to 1Ds users moving down for more megapixels.
3D - 30-40 megapixels, 7D AF, 3-4 FPS, 100% viewfinder, pro build/sealing. - $3500-4000
5D mk3 - 18 megapixels, 7D AF, 5-6 FPS, less than 100% viewfinder, current build - $2000-2500