I've written something like that as statusbar applet for the Linux Desktop and OS2008 that screenscrapes my cellular operator's website and displays the data/voice usage:

http://thpinfo.com/2009/naranjito/

This currently only works for one operator, but could of course be extended to operators which allow the traffic to be seen from some (login-protected) website. Of course, modifying it to be a desktop widget should not be too hard, also...

The code is open source and available from the website.