Yeah, just happened to me to. All of my twitter followers were dropped, only email followers are counted now in the “followers” count on the blog homepage.
I assume what happened is that they were trying to fix the bug that was double-counting the twitter followers for many blogs. And they went too far and created a new bug whereby twitter followers aren’t counted at all.
Oh, and now I’ve noticed that the numbers for twitter followers have vanished from the stats page to. Not that there’s a zero. “Twitter” isn’t even listed as a category of follower on my stats page anymore.
There’s also another forum thread on this issue, which I guess has been identified as a bug. Some people are complaining their Twitter and Facebook numbers are being artificially inflated. Me, I’m seeing the same problem you are – – the Twitter followers have disappeared. I’m presuming WP is looking at this, even though the other thread has been marked resolved? Will watch for info.