As the author of a book about Twitter, I get a lot of questions about Twitter syntax. You may have noticed some people you follow in Twitter beginning a tweet with ".@" and wondering what it means. Here's an example:
Adding a . in front of an @ symbol at the beginning of a tweet will make that tweet visible to your followers in their timelines. Otherwise, if you don't add a period in front of the @ symbol, it's treated as an @reply, which will not be seen in their timelines unless they are also following the account mentioned. In the above case, if Scott Kirsner didn't put a period immediately before @graiz, the only people who would have seen it in their timelines are people who happen to also be following @graiz.
Confused? I don't blame you. @replies are visible in search or when people click on your timeline, and when a Twitter name is mentioned anywhere else in the message in any scenario it will show up in your followers' timelines. But Twitter wanted to make sure that @replies were treated as Twitter syntax. At first, it was merely a convention that users adopted, like RT. The Twitter blog explains the convoluted tale ...
By the way, you can follow me on Twitter at @ilamont!