I quote the legal contract when you win:
3. Transfer of Ownership The designer/vendor, upon purchase, thereby irrevocably transfers all rights, ownership, intellectual property interest and legal title of the Logo to the buyer. The designer/vendor will not use the design of the Logo in any further or derivative work (except for display as an example of the designer's work for portfolio purposes except as indicated herein) nor offer to resell the Logo to anyone else as the buyer obtains sole and exclusive ownership to the Logo via the purchase: it cannot be resold. If the contest was "Private" (an upgraded feature where only registered designers can view or participate in the logo contest), the designer/vendor must seek either written or email permission from the client to display the winning logo in the designer's portfolio.
I think that if you sell a logo in a Logo Contest you can show it, but if the contest was a private one then you have to ask the CH for permission.