What is your iphone doing while idle???
I use DataMan to track my 3G usage. One of the interesting things I learned from DataMan was that my iphone is using several MB of data all the time in the background, even when I'm not using it. While I'm sleeping, last night my iphone used 4mb of data each hour. DataMan will show you which apps are consuming your 3G, but not which apps are using your wifi. I would very much like to know which apps are using wifi. Are they sending out my personal data whenever I have a wifi connection?
The other reason I would like to know is I have a FreedomPop hotspot which gives me 500mb of free wifi data per month. This is a great deal, but this data will go fast if my iphone is tethered to the hotspot and it starts sucking up 4-5mb per hour when I'm not using it. Even paid hotsposts have data limits.
If you have DataMan, poke down into your daily usage and then hourly usage and see if your iphone is also consuming many mb of wifi when it's idle. I guess I'll have to buy WifiMan and find out. I would have been nice if the DataMan app reported this information for both 3G and Wifi, since they have the same developer.