My Captivate is a data hog and I have no idea why. I am talking hundreds of MBs in a few hours, without me using the phone. It then can go weeks with moderate use (<200MB/month). The network activity can get so high that my home DSL network gets bogged down. Turning on the phone's airplane mode helps the situation.
I am trying to figure out which application/service is causing the problem. The phone is receiving the data, so it could be a background synchronization issue. I wish there was an application that could get me application specific network metrics.
Right now, I have 3G data turned off.
Oh, I have WiFi at work and at home, but I am amazed at what the phone uses in my drive between the two places.