Unfortunately RIM has missed the most obvious feature in their phones. But there is a program I use, called Mobile Data Alerter.
You can set it to alert you when you are about to reach a threshold of data. It shows you exactly how much data you have used.
It's a free download!
Last edited by amazinglygraceless; 05-11-09 at 07:21 PM.
Reason: Added link for application