I'm not too sure how much "Solved" will really be helpful.
There will be a lot of posts that no admin has seen are solved, so they won't get marked.
Even if they are marked, until someone reads the whole post they won't know if the post actually contains a solution to the *exact* same problem as they have, just due to the nature of some of these problems and how specific they can be.
It kinda sounds like a request that a lot of extra work be done that I'm not sure will prove as beneficial as it seems like it would up front.
I mean, probably somewhere between 50-75% of posts here either aren't problems (announcements of new themes, etc, discussion of new ports so it will never be solved, so on) or are issues covered in the manual (so that they were "solved" in a very obvious place long before).