Here is a definative answer from the FAQ list of 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.