SMS and IM
SMS and IM: "Sure, you have presence, and a roster, but the store-and-forward mechanisms of SMS enable deferred delivery, delivery reports, and, above all, they don't impose on you. No "typing" notifications or popups to build awareness that the other person is waiting for your reply - in fact, there's no real pressure to reply instantly (at least not culturally, and not in my age range here, although I understand that younger people delight in ping-ponging dozens of inane messages between each other)."
Weirdly, I was chatting to a guy from a VC earlier today about exactly this. I'd never thought of it before, but I suspect usage of SMS and IM are rather different - one seems significantly more synchronous than the other. Can one replace the other - and will users want them to? I can't exactly see them running side-by-side...?