Twitter till now have no revenue model. However, there are third party applications available, which are promising, advertisements on twitter platform. There are three kind of models available for marketers to advertise on twitter.
CPM model of advertisement, through Be A Magpie. In this auction only the highest bidders ad gets triggered at the rate decided by the second highest bidder, for the same category. So, if you are bidding through this model, you need to bid very high for your ad tweets to appear. You have to prepay and there is no guarantee your ads would show up till you continuously try to bid more and more. Once you pay for it, keep checking the bids.
New CPC model of advertisement offered through twivert, cashfortweet and perhaps many others. In this case you pay per click, and there is fairly higher chance of your tweet ads being displayed. You need to sign up through twitter ID and they have O auth too!
Directly contact few Highly influential twitter account holders and ask them to tweet about your product. Recently Jeremy Shoemaker of shoemoney fame reported having been approached by an advertiser. Best for marketers if they can do it, however, twitter account holders, Beware, your reputation might be at stake if followers detect it.
Twitter's TOS prohibits automated tweeting which are intended to spam the system, and perhaps the first two are in violation of it. Third one is a difficult one and if you can do it, you are plain lucky. Great Twitterers, have great followings because of their integrity and if they start serving deceptive ad tweets, followers may not take it kindly. Advocacy may be an option, the silent the better.
All the models above compromises the quality of tweets. Won't it be better, for twitter and twitterers that twitter opens up the system for advertising, may be even through Google adsense's link units in tweets to end all these Not so Ethical looking practices.