My data usage on AT&T from Feb 2011 to Feb 2012. In August 2011, I sold my first generation iPad 3G ($30/month unlimited data plan) , got an iPad 2 (Wi-Fi only) , and added the tethered data plan to my iPhone 4 contract. AT&T upgraded my 2GB to 4GB when I added tethering.
Unless you are travel a lot for business , I don't think you need unlimited data. Do yourself a favor - log into your cellular account and take a look at your one year data usage.