Indeed, women do seem more likely to be guilty of this vocal tic than men - and there are lots of theories as to why. Perhaps it is a subconscious hedging - we know that authoritative speech is considered too aggressive for women, so we make statements sound like questions. Or maybe we use upspeak because we're fishing for confirmation that our audience is still listening. Or maybe we're just perpetually practicing to be on Jeopardy - phrasing our answers in the form of a question.
I have another theory. A graduating elevated pitch indicates an expectation. We end questions on high notes to indicate we expect a reply - and we often end rhetorical questions on a low note.
Pay attention to your pitch the next time you read out loud to a child - you'll notice that your voice wants to go up at the end of each page except the last, and does so more sharply at the exciting parts. I noticed myself doing this recently while reading to an antsy one-year-old - the more apparent it became that he was losing interest, the higher the pitch of my voice became. I think that's why so many kids use upspeak. They perceive that adults raise the pitch of their voice when they want to keep audience attention, so they model the same when they want someone to pay attention.
This same sort of "something is coming" tone is used in TV scores. Swelling violins to tell us a climax is coming. Progressively higher-pitched noises during scary horror movie scenes.
.The TV show Lost frequently had a discordant glissando right before a commercial break to manipulate us into staying tuned. You can hear the same glissando on strings in the end credits.
I'm curious whether this is the case in other languages. What say you, polyglots? Do questions go higher pitched in your language?