Here's a Twanalysis that was bound to appear sooner or later, but one that I nonethesame found both timely and amusing:
"What does it mean when hundreds of third party services (with questionable, if any, business models) are dependent upon a single service which itself has no business model?
"...Neal Wiser suggests that Twitter’s amazing growth and popularity are indicators that the company is at the center of an emerging Bubble and examines the risks and rewards that a bubble could present to the service." Full post here (via @Medieskugga). Come to think of it, I should rephrase my first line a bit:
Update 10/3-09, 13:51CET: a few days after I wrote this, Techchrunch's Michael Arrington suggested Twitter is going to build its business model on search (via Jackie Danicki), which was followed up by suggestions Twitter will start serving local news to users (via David Black) - all posts well worth reading.