I asked this question last year on the Transworld Yahoo reflector and was assured by another user that it could stay outside just fine. Still, seems to me--and especially if you have the model with the remote band-change capability (I don't)--you would want to do some waterproofing on the matching unit.
I don't use mine regularly, just when I'm at the beach. I was concerned about the sprinklers at the condo that come on every night and the sudden thurderstorms that blow up down there. Considering my investment in the antenna, I took it apart each evening, put all the parts in the shrubbery, and took the middle part--with the matching unit--upstairs with me. It only takes a couple of minutes to put it back together.
I know that is not an option if you use the antenna at home every day. Maybe someone will stop by with the definitive answer...
(PS: I still can't believe I paid this much for a portable antenna, but the darn thing works very, very well.)