Think Tank: Just Like, Er, Words, Not, Um, Throwaways : "... By far the newest — and most controversial — idea comes from Herbert Clark, a psychologist at Stanford, and Jean Fox Tree, a psychologist at the University of California, Santa Cruz, who determined that speakers use (and listeners understand) uh and um in distinct ways. Uh signals a forthcoming pause that will be short, while um signals a longer pause, she said. Uh and um are not acoustic accidents, but full-fledged words that signal a delay yet to come. Of course that is not necessarily a good thing in public speaking. "It makes you look weak when people have come to hear you prepared, and you're not prepared," Mr. Clark said. ..."
and further on... " ... But it may be Nicholas Christenfeld, a psychologist at the University of California, San Diego, and other researchers who have come up with the most appealing findings. He counted uhs among professors giving lectures and found that the humanities professors say you know and uh 4.85 times per minute, social scientists 3.84 and natural science professors 1.39 times, which, he said, suggests that humanists have more expressive options from which to choose. ..."
Well, uh, hmm... I find this interesting. ;)
Seriously.. the use of pause words and their function is something we obviously learn and then place layers of meaning on top of those. The pauses in and around certain topics is something counselors attempt to interpret, but it is always contextual. Culture - both broadly defined (as in the UC study) and the more narrow definition (as in the opening paragraphs of the article) seems to play a very large role in the specifics of these pauses.