A really extensive list of brands utilizing twitter can be found at this location, called the twitter brand index. A lot more, than I thought, and a lot more US-focused (for obvious reasons) than I would have liked.
This at least solves the question: is twitter already relevant for marketing? It does not yet solve the question: is it effective for marketing? But that question will probably only be answered much later, once the user base of twitter has reached a critical mass.