Until someone does the hack, you can accomplish this with using 3 or 4 different browsers. Each browser logged into a different persona. It could easily look like they are all on at the same time if it's important to have the appearance of truly seperate people. Using the different browsers also makes it easier to keep your different personalities straight.
IE, Netscape, Mozilla, Opera, etc., all use different cookies and thus can all log in as seperate people. Just a thought.