Groups clearly need to be included in the platform, like in FriendFeed. As a second best choice, clients will talk to http://socialwhale.com/. Its twitter clone but extended API could potenially fill the void until, unavoidably Twitter meets the demand. I would leave interoperability to be the third choice, although maybe the most viable for now (if I remember correctly, SocialWhale has already announced TwitDeck groups import).

It reminds me of the endless FFd wishlist until FFd met many of them with their new UI. Either twitter will follow or, yet again, suffer from its own success.