Thanks again to all for the continued input.
Here's where we're at:
My mate and I discuss bike types for the NA trip on a daily basis now and we have reached the conclusion that we definately don't want sports, supersport or sport tourer bikes. It has either got to be a comfy road cruiser in the HD vain (but preferably Jap) or a mid size dual sport (like my XTZ). Reliability being the key - we aren't interested in lots of shiney crome bits and bobs, in fact, the more rugged and dusty the better!
The main reason we are leaning towards the cruiser end is that we have had (and will continue to have) plenty of off road trips in various countries, and with the NA road conditions the way they are, we figured that it would make a nice change to stick to the black stuff. The cruiser seems to be the best bike for this; for comfort, fun and style (I must admit, the image does have some appeal).
I have spent countless hours scouring the net.ca searching for possible machines and they don't seem as cheap as I had hoped, and we are seriously concerned about using up precious travel time bike hunting when we arrive. Consequently we may get a couple of XVs or similar here and do the shipping thing. I have seen a few of them around here for about £1500 which is a comfy budget for us.
My next concerns are about shipping agents and cost etc, but I will post that next set of questions in the shipping forums.
If anyone has any direct experience with traversing NA on mid size Jap (or HD, I guess) cruisers then I would be very interested to hear your advice.