I'm taking a look at Microsoft Fuse Labs' SocialGadget which provides a way to visually explore Twitter topics. I used the search term "H1N1." You can see the result in the screenshot to the left. It turned out that "Justin Bieber" and "H1N1" were tweeted together 19 times in the past 3 days. That was enough for that name to show up in the tag cloud. Go figure.