So for example, @rustybrick returns my "latest" Tweet.
The thing is, I took this screen shot five minutes after making a new Tweet. Here is that new tweet:
So we know that Bing must pull from the public Twitter API, not in real time, but on set intervals. A lot of people are touting this Bing/Twitter search answer as "real time." If it was literally real time, the Bing search results that contain tweets would have the potential to fail too often. Twitter is known for their downtime and to pull Tweets in real time from the API can fail also. So clearly, Bing is storing these Tweets in a local data source and serving them up in close to real time, but not exactly real time.
FYI, it is now 10 minutes after tweeting and the tweet does not show up in Bing.
Forum discussion at WebmasterWorld.