Personally I beleive incidences of tip-up cutting or impalement are grossly exaggerated. I've been carrying clipped knives of various sorts for....about twelve years now, not as long as most 'round here, but long enough to have been around the block, and I've had one cutting injury, which was MY OWN fault, not the knifes' That one occurred when I put zip ties through the opening hole of my Axis AFCK and didn't trim them worth a damn, the sharp edge of the remaining zip tie caught my pocket, it opened, and I got a small cut on my thumb, not even necessarily super glue time if I recall correctly. Bear in mind that was my own redneck engineering, not the knifes fault.
I have however had tip down knives work thier way open once or twice when wearing loose pants, no injury occurred there. since since I caught it. On these I chalked it up to just plain life. Not everything can be controlled in life, feces happens. During both incidents I was engaged in extremely rigorous activities,which compounds the likelihood of such incidents.
Being the reckless daredevil sorta guy I am, I prefer tip up carry, but I won't turn a good knife away cuz it's tip down either. I really think it's a kinda overblown debate in alot of ways. With lawyers running around and class action lawsuits being so trendy, do you really think they would make knives with clips at all if either method were a serious issue? These knife manufacturers aren't dumb by any stretch of the imagination ya' know...
If it were to come up as a problem, first thing I'd look at is the blade itself and how loose it is. I suppose I could imagine such incidences coming from two bit China clones and the such.