[John Beresford Tipton]

"Has anyone ever been successfully sued for a comment made over the internet on a message board or discussion group?"

Oh, yes.
A lot of successful litigation related to corporations, e.g., by ex-employees mouthing off.
Also stock chat boards where "opinions" have been deemed to be stock manipulation.

[Chancellor Palpatine]

Yep, 'tis all too true.