Paul Thurrott reports that his WP7 phone sends "huge chunks of data seemingly in patterns." Prompted by an AT&T email that he was close to his 2 GB data limit, he found that his data usage consistently increased over the months. According to the BBC, other reports claim that WP7 phones send between 30 and 50 MB of data every day and have idle data usage between 2 and 5 MB per hour.
If that is true, then we hope that you are not stuck with AT&T and especially not with a 200 MB data plan. Using a WP7 phone could get rather expensive. Microsoft said it is investigating the problem and release a fix, if necessary.
Microsoft CEO Steve Ballmer announced at the recent CES 2011 that WP7 will get a copy-and-paste upgrade as well as performance upgrades that will accelerate the launch of applications later this year. It appears that one more upgrade has just been added to that list.