Microsoft is apparently investigating so-called "phantom data" use spotted by Windows Phone 7 device owners, who claim that smartphones based on the platform continue to gobble down 3G data despite being connected over WiFi. The company told the BBC that they "are investigating this issue to determine the root cause and will update with information and guidance as it becomes available"; some users have found their handsets transferring as much of 50MB of data each day, despite the phone not being used at the time.
Various suggestions have been made about the reasons for the background activity, including the idea that Windows Phone 7 is quietly reporting usage statistics to Microsoft. It's not unusual for smartphone OS' to do so - usually there's a privacy option in the settings menu - but if this is the cause then it seems Microsoft may have to tweak the amount of feedback they're getting.
Alternatively, others have suggested it is simply a problem with how different connections are prioritized, with some processes not switching automatically to WiFi when it is available.