It's actually a pretty common question. At the bottom of the thread, where the reply box is, there is a large yellow button "Reply to Thread". Next to that button there is a link that says, "Mark as Solved" Click it and then voila, the thread is now solved.
Alex Edwards commented: =P +4
peter_budo commented: Exactly my point +10
Ancient Dragon commented: Good suggestion :) +32
zandiago commented: Maybe I'm glad...who knows? -1