Twitter has announced that it's going to start respecting the Do Not Track header. That's because it plans to start tracking people across the web, via its buttons, widgets, embedded tweets and so on.
But before it can do that, it has to update its Terms of Service
Learning a thing or two from Google, Twitter is not making much fuss about it and it certainly isn't going to be notifying all of its users across all of its apps of the change. It did tweet about it though.
The changes reflect the change in "widget data" collection. Twitter now tracks and stores data gathered via widgets, i.e. tweet buttons, follow buttons, embedded tweets and so on.
You can opt out of being tracked and the data is stored only 10 days anyway. Data derived from your visits is used for tailored content, which is kept indefinitely, but is tied only to your cookie ID.