GamerGod has published a preview of Cryptic Studios' City of Villains, based upon what we currently know about the superhero MMORPG expansion. A paragraph to follow:
Unlike most traditional MMO expansions, City of Villains is a stand alone product. While this is a great new concept, there are clear advantages to owning and playing both. Without City of Heroes, your only option will be to play a villain. Likewise, City of Villains owners that do not own City of Heroes will only be able to play Villains. As I mentioned their will be lairs in City of Villains and this is one area of where the features overlap. People that only own City of Heroes will be able to build hero bases. For the most part however, you will need to own a copy of each game to take advantage of what each game offers.