Blizzard just added two new features to the armory page. One of them is the browser plugin I was talking about yesterday. Apparently they made a plugin not only for Firefox but for Internet Explorer and Opera as well. You can find that in this link.
The other feature is the arena calculator. This isn't the first arena calculator around. But this one is the official one. It's also easier to use. It shows you how many points you'll get for the week given your rating for all arena categories. It will also show you how long it will take you to get a certain arena item you want given your current rating and how many arena points you currently have. As I said, this isn't the first of it's kind but this is the clearest and easiest to use. The information is straightforward and it doesn't leave me confused.