This question has been answered in the newsgroup many times, in varying amounts of detail, and a number of extended discussions have resulted from it.  Instead of rehashing the entire debate, here's the short answer, excerpted from -

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html

"... 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. "

Check the website for a more in-depth answer, and links to other sites dealing with this question.

Here's another site -

http://www.straightdope.com/classics/a3_147.html

with a slightly off-beat, but accurate answer to the question.