I've written a userscript for Firefox that displays MB/Day under Included Data and Remaining Data.
It's useful if you're worried about going over your cap (as I tend to do often), and gives you some idea of how much data you'll be able to download each day.
Now, it SHOULD work if you have more than one connection in the list, but I only have one and have written the script under assumptions of how extra connections might be added to the page.
[Firefox] Install the Greasemonkey add-on, then just drag airnet.user.js into an open window to install.
[Chrome] Open your extensions menu then drag airnet.user.js onto it.