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