I've learnt more from the solved threads than anything else on this site (manual and tutorials included), so I'd strongly disagree that they should be removed. Going into a thread and finding that it's solved usually prompts me to see how it was solved, and add that little nugget of information to my own repertoire.
Moving them to a sub-forum would mean that they'd be neglected, relegated purely to searches. I enjoy reading through all threads in this section and if they're put off to the side then it'll just make it more of an effort to browse through them.
A "solved" tag, on the other hand, wouldn't be remiss.
Actually, giving it some thought I'd really appreciate a tag feature around here, and I think it'd be especially helpful for the newcomers. Tags like beginner, intermediate, advanced, physics, behaviour, UI and so on would be very useful to organise content at a glance, and could make searching a whole lot more powerful, too.GeometriX2013-02-02 18:38:30