I've never had anything stolen but we don't leave anything out in plain sight either.
We always keep our extra cash in the safe (we don't have that much as we use our KTTW card for everything), we also keep our passports, Nintendo DS, ipod, etc. in the safe as well - that's about all that will fit actually!
We also keep anything else of "interest" in the suitcase and we always zip it up before we leave the room. If anybody really wants to take something I guess they'd go looking but I don't want to to tempt them!