It'll be interesting to see what manufacturer, if any, produces a combo finder system.
I can think of four ways to do it, with current or near-future technology:
1. Interchangeable finder heads, as Bjørn has described: Probably the simplest, most elegant solution from the standpoint of the operator and camera compactness, which is good. A possible downside is future compatibility: Does the D6 EVF work on the D7?
2. Non-interchageable combo finder: No switching out of finder units, but will probably increase size and mechanical complexity beyond what the customers would want---and want to pay for---even though, on the top pro models, compactness of design is the last consideration of the designers. Imagine a D5 with something the size of a Photomic FTn finder bolted to the top. Expensive, and therefore a hard sell.
Could a compact version, with a transparent(when not being used) EVF be created, as Sten has suggested? Maybe...but I'd fear that the EVF image quality, which already has problems in general, might be compromised further.
3. Clip-on auxilliary EVF: Has already been done, but current attempts have been plagued by cheapness and poor ergonomics. Not throwing off the handling balance of the overall camera would be the biggest challenge. This solution would allow the additional possibility of a real, 170º swiveling eyepiece in the style of video cameras---a built-in right angle finder that does more than right angles.
4. A quick-attach, rear-viewing loupe that utilizes the rear LCD, but snaps onto the body quickly, securely, and with no addtional rigging or bracketry. Bonus points if they can make it swivel....
