Twitter today will announce its grand plan to cash in on its status as a global phenomenon – by allowing you to sponsor tweets.

Called Promoted Tweets, the program will make official (and for a fee) what businesses already have been doing for a while: Promoting their brand through the microblogging service.

Don't worry: you won't now be charged for using Twitter just because you're a business, or at least, not yet. Sponsored tweets will just get an extra push to help them be heard above Twitter's 600 tweets per second, and, the company hopes, keep them from immediately being pushed out of the way by wave after wave of 140-character missives. Users will be prompted to re-tweet advertisements, and the tweets (which will still be limited to 140 characters) will wend their way to users' Twitter feeds. The appearance of ads will be targeted – advertisers will buy keywords, and the Promoted Tweet will show up when a user searches for that particular term, much like the way ads appear on Google. (Unlike on Google, only one ad at a time will appear in a users' feed.)

If this all sounds a bit Groundhog Day, it's because it is: Early Monday search-ad pioneer Bill Gross unveiled TweetUp, a Pasadena start-up that allows marketers to buy keywords to promote their tweets. TweetUp's formula also ranks tweets on factors like the writer's expertise and popularity, and the paid-for tweets will appear atop filtered searches on Twitter applications such as Seesmic and TwitterFeed, which account for 80 percent of tweeting activity. (Can IdeaLab founder Gross turn the launch of Promoted Tweets into good news for his fledgling company? Stay tuned.)

First in line to try Twitter's new service: Starbucks, Bravo, Sony Pictures, Best Buy and Virgin America, all of whom are huge Twitter users already. Between 2 and 10 percent of Twitter's 45 million regular users will see the first ads today.

In the past, Twitter's founders have said they wanted to concentrate on growth and not alienate users. But in September the site amended its terms of use, paving the way for advertising. Although it's valued at $1 billion, the site made almost no revenue until it began selling real-time search results to Google in December. (In contrast, analysts say Facebook will pull in $1 billion in revenue this year, largely thanks to demographically targeted advertising. Should you advertise there? Click here.)

What remains to be seen is how users will respond to advertisers in effect butting into their conversations uninvited – and Twitter, of course, hopes it has a solution.

Promoted Tweets "must meet a higher bar – they must resonate with users," Twitter co-founder Biz Stone wrote in a blog entry titled "Hello World" and posted at midnight Monday. If users don't reply to paid tweets by replying to them, favoriting them, retweeting them, or clicking on embedded links – thus helping a particular ad achieve a certain "resonance score," --  the missive will disappear. The company won't have to pay for it, and users won't see it anymore.

In the beginning, companies will pay per thousand people who see the sponsored tweets, which will disclose that they are ads, and will turn yellow when moused over. Once Twitter gets a handle on how (and if) people interact with the posts, it will roll out alternate ways for advertisers to pay for play.

"Is it great in the search and horrible in the timeline?" Twitter Chief Operating Officer Dick Costolo told Ad Age of the sponsored tweets. (He'll formally announce the advertising scheme at an Ad Age conference in New York today.) "We are going to test and test and test." The company will decide in the fourth quarter whether to take ads into users' Twitter feeds.