At the moment Blizzard forum moderators are apparently deleting all threads pertaining to the ads, which seems appropriate enough; they clearly know about the problem. Oh, and folks: don't buy gold.
Update: Blizzard has responded officially to the matter, stating: "Recently, there was a temporary error with our forum advertisements that caused a gold-selling ad to be displayed. At this time, we have resolved this error. Our stance... remains firmly against the buying or selling of gold, and we may take action against accounts that are involved in these activities."