It's been whispered about and openly discussed at XDA for some time, but it has now been confirmed that Yahoo! email is the so-called "data hog" culprit on Windows Phone. To refresh, some users were reporting large amounts of data traffic being sent from their phones, resulting in some people approaching or going over their capped data limit. Yet others saw no such behavior. Microsoft finally investigated and found the source themselves but refused to name them publicly, instead they tried to address it behind doors.
Now Raphael Rivera, part of the ChevronWP7 team, has gone ahead and created some sophisticated tests to nail down the offending app. Yahoo email has been suspected by many for some time (see here and here) but now seemingly concrete proof has been demonstrated. To sum up the technical by Rivera, Yahoo appears to be sending around 25 times the amount of data that it needs to, which is quite an increase. As a result, Rivera recommends the following for Yahoo! mail users:
To workaround this, I strongly recommend Yahoo mail users reconfigure the phone to not transmit data via a cellular connection (Settings –> Cellular –> Data roaming options). As an alternative, you can set your Yahoo account to only Download new content only on manual trigger (Yahoo Mail –> Settings –> Sync Settings).
Sounds like sage advice. Seeing as Yahoo is the culprit here, this seems to explain why some of us did not ever experience such behavior, while others did. This also means that it's not WP7's fault but rather something on Yahoo's end that needs to be addressed. Read more on the nitty-gritty on Rivera's page here.
Update: Microsoft is now officially acknowledging the issue: "Microsoft and Yahoo! have worked together to identify a fix, which will be rolled out in the coming weeks.". There also is a rare problem with Exchange Active Sync (EAS) which should be fixed in a software update.