Is there a plasmoid or other GUI application that measures internet data usage over a period of time? I use my mobile card quite a bit and I want to be able to watch my total data usage without having to log into my mobile account frequently. It doesn't need to be accurate down to the bit, but a warning before I'm 10 mb deep in data charges would be nice.

Whilst this is currently not possible in KDE SC 4.4, you may wish to check kde-apps.org, as someone may have written a Plasmoid which does this.