As one of wp7Central editors wrote earlier, the so-called data usage bug in Windows Phone 7 is evidently not a bug with Windows Phone 7 at all. According to Microsoft, who put substantial research into the matter:
We have determined that a third-party solution commonly accessed from Windows Phones is configured in a manner that potentially cause larger than expected data downloads. We are in contact with the third party to assist them in making the necessary fixes, and are also pursuing potential workarounds to address the configuration issue in case those are needed. At this point in our investigation, we believe this is responsible for most of the reported incidents.
Unfortunately, this response does not give a significant amount of information on what Microsoft proposes the problem is, as it leaves Facebook and email integration as another possibility. To reaffirm, however, Microsoft states:
A small (low single-digit) percentage of Windows Phone customers have reported being affected.
We are continuing to investigate this issue and will update with additional information and guidance as it becomes available.
If you're encountering this problem, we suggest you diagnose the problem by disabling certain services, but without more information, the affected users are left by Microsoft to sit this one out.