With a few people reporting issues with excess data being sent by their Windows Phone 7 over 3G (see update) an interesting question came about how to monitor your data traffic coming from your device. The typical method is data sniffing but traditionally this has been beyond the reach of most non-techies.
Eric Law on his MSDN blog has detailed a method using Fiddler to watch your phone's data traffic over WiFi (so this won't help with that 3G issue). Primarily this is a great tool for developers to make sure their app is not using excess data in any way but it can also be useful for those who just want to know what's going on.
For the record, we had some initial trouble getting this to work on our PC but you may have better luck as the process is pretty simple and straightforward--maybe 15 minutes tops and that's including reading. So give it a shot if you're curious and let us know if you learn anything cool.