According to Microsoft software ‘deconstructor’ Rafael Rivera, he does have some ground to believe that the reason several Windows Phone 7 devices actually sneak in massive amounts of ‘phantom data’ is due to an issue with Yahoo Mail. Rivera claimed that he looked through the IMAP packets that leave the device, noticing that “Yahoo’s IMAP server was not responding to the phone’s requests to fetch new messages with the correct response.” Hence, in lieu of sending tiny commands, the servers started to return blocks of data up to 25 time larger than required. Basically, your smartphone will begin to download around 25 times more data than it normally would. Neither company has issued a comment on this development though, but Microsoft did mention last week that there was only a small (low single-digit) percentage of WP7 owners who were experiencing the data leakage.
Update: Yahoo has issued a statement concerning this issue.