I'm not sure why this hasn't been done earlier, but it looks like I'm the first to integrate xPL and Twitter. It was really easy to do and only took a few hours. A lot of the time was just figuring out what API to use and investigating OAUTH. I ended up using twitterizer's API, which doesn't support OAUTH. That's fine because I want this app to run standalone without any user interaction required for login, which OAUTH requires. The downside is the tweet source will show Twitterizer and not xPLTweet. Twitter no longer takes registrations for apps that do not use OAUTH, so there's no free publicity for my app in your Twitter streams. Bummer.
xPLTweet grabs OSD.BASIC messages explicitly targeting it (ie doghouse-xpltweet.yourinstance). It grabs the text portion of the message and tweets that. That's pretty much all it does. A future enhancement I may do will be to monitor who you're following and issue OSD.BASIC messages when they update their streams, but it could be messy if you follow many active Twitterers.
In any case, you can grap xPLTweet here.