Of course, that's just plain not true (yes, I'm getting the "last word" in before we even publish the first word). Want evidence? Look no further than Web Worker Daily's Real-Life Twitter Business Success Stories. Here's an excerpt:
DJ Waldow of Bronto met a prospect through Twitter who knew of the company and valued the company's industry insight. When it came time for the prospect to find a new vendor, he went to Waldow and turned into a client. Bronto built a relationship with the prospect through Twitter, which helped the company stay in the prospect's mind.
Granted, there's nothing here from the likes of Coca-Cola or Microsoft; we've yet to see an example of a big business really capitalizing on Twitter. But I'm of the belief that'll happen soon, after which the marketing-via-Twitter floodgates will open wide. What do you think?