This totally happened to me! When I switched from the iPhone 1st gen to the iPhone 4 I went with the 2GB plan plus tethering. Half way through the billing cycle, my data use jumped up to over the 2GB limit (I had been watching my use close so I would not go over.) One day I was near 1GB (close to half use @ half-way through the cycle). Then the next day I was over 2GB! This happened over night. Sure enough, when I looked at the detailed use on ATT's website there were erroneousness data packages sent between 1am and 4am (I was very much asleep) Each data packet was around 200MB each! When I called ATT could not explain these data spikes. I had not changed any applications so they couldn't blame it on an application. After working my way up the chain of command, I finally spoke with a manager of some sort that said they could not explain where the data was coming from, but they would just give me the unlimited plan back and it wouldn't matter where the data came from. I haven't watched my data as close since then, but I notice that the bill always has a higher data amount than the data recording within the iPhone preferences states. I really want to switch to Verizon now!