I don't know what the 5D Mark III will be like, but if HD video is your focus, the Nikon D800 goes a long way towards an affordable full-frame video solution. Unlike Canon, Nikon has more interest in gearing a DSLR to your video needs in lieu of pushing you towards a professional video camera line; they haven't had one since the Hi8 days. Only downside is that it may not have PAL support initially if your target audience is in Europe.
If you can excuse having to buy a special adapter to get audio monitoring and don't need 60fps, the Unified Branch of the community-driven Magic Lantern firmware effort
will probably have a stable release for the 5D Mark II soon, giving it some much needed video love, and with used 5D Mark II's going for ~1800 USD, that's *really* affordable. Magic Lantern's manual focus helpers are probably better than you'd find on even the D800.
My fingers are crossed for a stellar 5D Mark III. Maybe it'll have an equivalent to the 1DX sensor as you hope.
If your definition of HD is 4K+ resolution, you'll probably not see anything close to that from HDSLRs in 2012.