Swap publish date with revision date?

Is there a plugin out there that changes the post “publish date” to the date/time it was last revised? Since most posts are sorted by date, I need a way to make the freshly updated ones rise to the top, so to speak… These arent actually posts but rather “business listings”… We want to encourage business owneres (post authors) to update their biz listing (post) often. Then, any time they put up a new special or coupon they would be at the top of our “featured business” slider on the home page. Any help out there?