While I balk at using Twittering for marketing, the fact that this doesn't make money for anyone makes it okay. In my opinion, the only people who should make money out of twitter are the owners of twitter.com (and they're not yet.) It somehow feels wrong to turn a community into a marketing machine. I, for one, will bail from Twitter if I sense the winds blowing in the marketing direction.
I'm not saying people shouldn't make money. Far from it. I see Twitter as something more than just another Web 2.0 application. There's a community building there, and I'd hate to see it turned into yet one more way for people to make money. Casually mentioning your products is okay. If people follow a person with products, then they have to expect those mentions. But out and out marketing on Twitter is an evil I don't want to comprehend.