Nova Media has released a program for Mac that makes it a little easier to monitor your data usage on mobile connections and allows you to set up alerts to warn you when limits are being reached.
If you want some help making sure you don't exceed your data plan limits, Nova Media has released a statistics manager, Traffic2Net, that keeps track of time and data usage on your mobile Internet connections. You can see how long you have been online and how many megabytes of data you've downloaded, and even set up pop-up alerts to warn you when you've reached certain limits.
The website states that the program works with "2G/3G USB modems, ExpressCards and PC-Card modems: Traffic2Net works with most mobile Internet devices around the world." The price is around US$15, but there is a free demoversion of the product that you can try. It requires Mac OS X 10.5.x, 10.6.x.