The short answer from Matthew Yglesias of
Talking Points Memo is, not very.
Essentially, you've got a lot of people running around screaming and yelling "Aagh! We're all gonna die!" but Iran is spending only $6b a year on its armed forces, compared to $9.6b for Israel and $25.2b for Saudia Arabia. Plus, it might be a good ida to keep in mind that Iran is surrounded by American friends and American-occupied countries.