Twitter previously developed a Facebook app for users of both services that takes in a user's Twitter content and posts it on Facebook. Apparently a lot of people installed it but left it turned off, and now Facebook is, in error, ignoring the setting and letting the app do what it was designed to do even if users don't want it.
There are also reports that the same thing is happening with the Friendfeed app, which quite complicates things since many people give Friendfeed itself access to their Twitter feeds.
Consensus seems to be that a Facebook bug is causing the platform to ignore users' "off" settings on their Twitter and Friendfeed apps. A solution is to block the offending applications (Twitter, Friendfeed) from accessing your profile.
Update, 11:40 PM: Facebook released this statement:
Earlier this evening, a small Facebook bug allowed a handful of apps to publish to the stream on behalf of users who had previously authorized the app. The situation has now been resolved, and all application settings will remain intact for users.