Hey, just discovered the "new and improved" Forum has this feature. (In two recent posts it underlined a couple of screen names, which of course aren't in the dictionary, but it also caught an unintentional spelling error.)
I personally hate to write posts with spelling errors as they make me appear even dumber than I really am. But, like most, I was too lazy to crack the dictionary. Of course it will not correct misuse of "there, their, they're" or "to, too, two," but still it is great for those of us who would like our posts as easily readable as possible.
THANK YOU to whoever added this feature.