In reply to Duz Walker:
> (In reply to CJD)
>
> Fall off a highball and the probability is a sprain or bruise but a fracture is possible.
>
> Fall off a route and death is possible (height or landing).
I think that's a bit too absolute and objective to work - you'd be unlucky, but it's theoretically possible to sustain a fatal basal skull fracture from a fall of just a few feet and with no direct blow to the head. Admittedly I've never actually heard of this, I'm just playing Devil's Advocate.
I was thinking about a subjective definition, sotheing like 'if you're prepared to take the fall it's a boulder problem, if you don't consider the fall unacceptable it's a solo'.
Then again, what about something like High Plains Drifter (sorry, don't boulder much so I can't think of a UK example). I certainly wouldn't want to fall off the top of that, but then it's only about 5.7/5.8 which is nothing if you've done the 'meat' of the problem.
I think it's a very blurred line, and probably has as much to do with the history and consensus style of ascent. There's no real answer. In that respect it's a little like the question of where the line between an easy solo and scramble is drawn - it's purely subjective.