I‘ve just read a case out of the Federal Court in Idaho (Alamar Ranch, LLC v. City of Boise, 2009 WL 3669741 (D. Idaho Nov. 2, 2009)) which held that emails sent by a non-party to her attorney using a work computer were NOT protected by the attorney-client privilege. The court further held that emails from the attorney to the work email account were also not protected.
This result stems from the company policy that employee emails were subject to monitoring, and were not confidential. (a very common company policy nowadays).
In fact, many companies monitor EVERYTHING their employees do using company computers and network connections, so even using a private, personal email account from a work computer might essentially waive any attorney-client privilege or confidentiality.
One other caveat, some online email providers and social media sites actually track the contents of messages so they can serve you targeted advertising. The question whether this also breaks the confidentiality of attorney-client communications remains open. This is why I, for one, do not use gmail, facebook, myspace or similar services to communicate with clients about their legal matters.
The only true way to be certain your Internet communications are truly secure is to use some form of encryption. Until encrypted email is easier to use, the ‘hassle factor' prevents it from being widely adopted.