Same problem here, Samsung Galaxy S II Skyrocket. We got four of them. My brother had this problem immediately after getting his. The only remedy for him was to switch out the phone. Fortunately for him, he was within the 30 days. His phone was sucking huge amounts of data while he was sleeping.
Now mine just started doing it. I got the text message from ATT that I have used 90 percent of my data, just seven days into the new cycle. All of the last nine months, I haven't even used half of my available 2 gigs.
ATT told me I used over 1 gig at 10:30 AM, while I was driving to an 11:00 AM job. There is something strange going on. I have not downloaded anything new, nor have I changed the way I use my phone in any way. It is set up to use the wi fi while at home.
This is crazy! ATT sure has to be aware that there is a problem. They were very nice and promised to take care of it, but that is unfair that now I am going to have to be super careful, unless I can get rid of this data hog.