BAH! I've had feedback on since day one and my data usage for the whole month of Dec was 512.5MB. According to my online account, with only 4 days left in my billing cycle, I've only used 389MB.
I think it's because people are downloading big apps that start on wifi but once the screen turns off and wifi gets shut off it falls on 3G for the remainder of the download.
Wifi does not need to be on to continue downloading a big app. It only has to start on wifi.