Like it or not, Twitter ads are coming
Great analysis & explication of Twitter's ad strategy by Peter Kafka, among which is this bottom line:
Important — will definitely expand into regular timeline, at some point. IE – you will be getting ads in your stream whether you search or not. Ad-free Twitter is over.
They're calling them "promoted Tweets," not ads. Seems a bit dishonest. Will be interesting to see how well Twitter distinguishes the real Tweets from the paid ones. Google is pretty good about this.