I didn't want to use the article's headline which I feel is a bit sensationalist:
"Reading someone's Gmail doesn't violate federal statute, court finds"
http://arstechnica.com/tech-policy/2012/10/reading-someones-gmail-doesnt-violate-federal-statute-court-finds/
It's an instructive read, though maybe not as the author intended... My summary, FWIW -- the article shows a bit of how the courts interpret the language of the law, as well as showing that existing U.S. laws [& very likely similar laws everywhere else], seem hopelessly outdated in today's world. While it's illegal [AFAIK in most places in the world] to hack into someone's on-line accounts, separate laws &/or regulations cover the ownership & use of any discovered content. What I thought particularly interesting was the way that on-line storage was/is interpreted -- the way I read it is that if you have 2 copies of whatever, the on-line stored copy is considered backup & thus protected, but if not, if you don't have another, probably local copy, it's abandoned & fair game for all.
Then at the end of the article there's this -- Orin Kerr, a privacy expert and professor of law at George Washington University says: "Internet providers often have a national customer base. A provider in one state or circuit can have millions of customers in any other state or circuit. Given the national customer base, any disagreement among lower courts causes major headaches: ISPs don’t know which rule to follow. Making matters even more worrisome, it’s not at all clear whether the legal standard should be based on where the litigation arises or where the ISP is located." That brings to mind issues with U.S. based companies [e.g. Microsoft], complying with US rather than EU privacy laws/regs.
Long story short, till all this stuff gets sorted out you're probably safest assuming that if it's on-line, it's public.