Fame is an imbalance of attention - previously it's been due to technology (you can shout at the TV all you want, they can't hear you), now it's because we've run up against the limits of possible human attention (if my blog is read by millions, I can't interact with all of them).
If attention is a currency, famous people are rich.
This imbalance necessitates grouping your 'friends' on social networks into subgroups, classes of relative intimacy, or perhaps just of relative attention.
Twitter should run up against this need soon, it's got lots of famous people on it (read: poor sods that can never read all the tweets that their audience^W legions of followers spam out). They should really add that functionality sometime soon, or the writers of Twitter clients will do it for them - and then we'd all be stuck with either the-one-client-to-rule-them-all, or a mess of competing grouping systems, and no way (until someone hacks up a web api anyway) to migrate cleanly between clients.
For famous people with a need for the power that a large audience of followers brings and the desire for a Twitter client that doesn't accidentally hide your best friends coffee invitation among '@brownbanana, you were SO shitfaced last night!'
Right now, either the famous don't friend you and the relationship stays one-sided (which would be running at odds with the great wish for the democratising power of the internet, and is not what I'm seeing), or they friend and get swamped. That must be interesting - post a comment and then skim through a bunch of replies.
Either way, they must be relying on the @reply, or the more intimate DM. Perhaps they reserve the DM for bestest friends..hm
The use of @replies may be self, or rather socially limiting - the need to conciously address a message to a famous person is somewhat stressful for most people (unless they're terminally friendly) and may be limiting the frequency of communication. We attribute greater social standing to the famous, and that means we value their opinion more than any random fool - perhaps because we talk to them so little (i.e. have so little of their attention) that we are aware they may gain a negative impression of us if we mess it up.
api must link between Twitter and the client they are most likely to use - I'm guessing that would be an iPhone one
#prediction, #business plan