The difficulty will be translating autofocus and aperture signals, and whether the camera will work at all if if can't talk to the lens properly.
I think I read somewhere that the camera can shoot without a lens attached, so any old lens should be OK provided it has focus and aperture rings, using the camera in A-mode and manually focussing in a bright viewfinder with magnification. That sounds nice to me - in theory I can do all this with my Olympus DSLR (I've got several adaptors for Zeiss, Nikon etc) but as mentioned above the 4/3 viewfinder isn't really good enough for manual focussing.
I think pricing is going to be the problem with this baby though. It seems likely to be a lot more expensive than existing entry-level DSLRs so the advantages are going to have to be
very real for it to carve a niche alongside say the mid-level DSLRs which are pretty desirable things in their own right.
Smaller size? - the G1 is not as small as it could be, a mistake in my opinion.
'Better' viewfinder? - well yes, I believe in that, but not everyone is going to buy into this concept.
4/3 compatibilty? - maybe a bit compromised and in any case is largely pointless - carrying a kit of Oly lenses just negates the whole 'small camera' thing. m4/3 works as a new system or not at all, and that's where it suddenly looks expensive.
Potential for mounting all sorts of other stuff is very interesting - but it needs in-body IS to make the most of that.