Here is a definative answer from the FAQ list of sci.space:

Newsgroups: sci.space
Subject: Frequently Asked Questions 12/15 - Controversial Questions

    HOW LONG CAN A HUMAN LIVE UNPROTECTED IN SPACE

    If you *don't* try to hold your breath, exposure to space for half a
    minute or so is unlikely to produce permanent injury. Holding your
    breath is likely to damage your lungs, something scuba divers have to
    watch out for when ascending, and you'll have eardrum trouble if your
    Eustachian tubes are badly plugged up, but theory predicts -- and animal
    experiments confirm -- that otherwise, exposure to vacuum causes no
    immediate injury. You do not explode. Your blood does not boil. You do
    not freeze. You do not instantly lose consciousness.

    Various minor problems (sunburn, possibly "the bends", certainly some
    [mild, reversible, painless] swelling of skin and underlying tissue)
    start after ten seconds or so. At some point you lose consciousness from
    lack of oxygen. Injuries accumulate. After perhaps one or two minutes,
    you're dying. The limits are not really known.