This Techcrunch article does a nice job of summarizing the problems at twitter, and someone at twitter wrote a decent blog entry the tries to explain what they see the problem as and make a subtle call for help by way of a challenge.
And I gotta admit, as a code monkey, I'm tempted to answer that challenge. ;) Only hitch is that the job's in San Fran, and I've got my own agenda to pursue. :P
Still, it sounds like they are complaining that the messaging system isn't one-message-to-one-recipient and how the relationships between users creates a rat's nest of dependencies. I've got to wonder exactly what it is about twitter that makes this a hard problem to solve, since IRC has handled the same problem since 1991.... the message routing isn't in the actual rails code, is it?