Twitter has started placing paid advertisements on twitter.com. Not that you would know unless you looked really hard!
The new, paid ads have been placed in the same place as the previous, non-paid ads (what Twitter call ‘definitions’ – see screen-shot.)
At the time of writing, there are just two paid ads like the one of the right, which rotate along with the original ones. The paid ads are clearly identifiable, with the words ’sponsored definition’ written below them.
Twitter has regularly stated that it is not going to pursue advertising as part of their business model. Surprisingly, this move has gone largely unreported and thus far, there’s been no word from Twitter regarding what looks like a U-turn. This news comes just two weeks after Nielsen reported that Twitter’s growth had slowed down dramatically.
UPDATE:
Since posting this, 
I received a message from Seth Simonds via Twitter about a post on his blog, which shows Twitter is already delivering banner ads to Japanese language users, see right. For the full low-down, check out Seth’s post!
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Are you crazy or dumb??? Of course this has gone unreported. Twitter have all the top news sites on their SUL. They give techcrunch mashable and revision3 etc hundreds of thousands of followers. Its not like their gonna say anything until they are told to.
Sheeeesh.
Oh Twitter…they’ll either figure things out or sell off and head on to other things with a cute stipend from whoever bought them out. (See Myspace’s Tom for reference)
Buy up the time and convert as many users as possible into avid readers by making a point to connect in a variety of ways, I say! =)
Seth,
Thanks for bringing the Japanese language ads to my attention. Much appreciated!!