Wow I guess I missed this thread all together, just getting in after four full pages of replies and exchange... I think solo tripping is, well, when you go by your self alone(dog is ok).
I trip a lot with others but I'm mostly alone in my canoe usually a tandem 16' tripping canoe, I don't consider my self a solo tripper, even if I the time on the water traveling is away from the group either ahead or behind them, sometime by as much as a few km...
I have friends(couple) that do remote trips of many weeks(4 to 8) every second years or so they always crave just the 2 of them, totally self supported and on crazy remote rivers(Thelon come to mind) and for some reason I think of them a solo trippers, maybe because they never ever trip with other people, they always only the 2 of them, I think of them as one! But that is the exception.
Interesting thread
Anyway, that is my take on it...