Windows Phone 7 users are complaining over what they call "phantom data usage." The nascent smartphone platform is, according to some accounts, using as much as multiple megabytes per hour of cellular data, and that's even when connected to wi-fi.
Indeed, you read that correctly; even when connected to wi-fi, Windows Phone
7 seems to be defaulting to cellular connectivity when there's a wi-fi network that the users's phone is connected to. In one report, a user stated
that his bill showed:
[...] on November 21-24 it sent between 30 and 50 MB of data at 10:41pm each day and Dec 1-4 it sent between 30 and 50 MB of data at 9:41am each day. On December 23rd I turned on airplane mode so my phone could no longer send data. I turned airplane mode off briefly on December 23rd and the phone sent 400 MB of data.
saw 2 - 5 MB used per hour. That would be about 1.4GB a month, with the phone just sitting on a table (assuming the 2MB figure).
This is probably a bug that will be fix with Microsoft's first big Windows Phone 7 update that should be coming very soon. Until then, WP7 users are seeing what happens sometimes to early adopters.