Why: For an ideas portal that has a lot of ideas being submitted, it would be beneficial to indicate you've viewed an idea. This would help a user understand they've already viewed an idea and decided not to vote on it. The only way to know that you've viewed something to date is if you voted on it. The absence of a vote means either you haven't voted for it but reviewed it or you haven't reviewed it yet. Without the ability to know you've reviewed it, I find myself spending extra time reviewing something I've already seen.
Who: All idea voters.
How: I don't think this should require entering the idea to select a widget. If I can tell based on what I see in the quick summary that this is something I would not vote for then I would just as much like to click a widget indicating I've viewed but chosen not to vote for it.