Since someone else bumped this thread, so I might as well put in my two cents...
I found an app called "Onavo Data counter." You tell the app your data limit for the month, it keeps a percentage track in the status bar. It also warns you about data hog apps (youtube, dailymotion, etc.) and lets you restrict them to wi-fi only. If something suddenly starts gobbling up data, it also sends you a warning instantly. Much better than the stock warnings from your service provider.
Since I got it, I've not gone over my sparing 500 MB data cap!