Had the same issue. It was my first time using cellular data on iPad and it ate through 50+ MB overnight with AT&T. I only got the 250MB plan so this really sucked.
I have since turned off iCloud's documents & data sync'ing so hopefully it won't happen again.
Indeed, so it will mightily suck when AT&T starts throttling you, just like they did with iPhones. And it will happen - mark my words.
I sold my unlimited AT&T plan and switched to Verizon. Speeds are double, and often triple, in my area. I'll clock in at less than 2GB monthly, and if I don't I can just upgrade to 5GB for 28 months with the money I made on the sale of my old plan
Also, I now have the luxury of canceling the plan for the stretches of time I didn't need it. With AT&T's slimy bait-and-switch, I had to maintain a continuous subscription in order to stay grandfathered in. Buh-bye!
OP:
I have the same situation.
I would often get the 1GB plan on my Verizon iPad 2 and it would easily last me the 30 days.
With the same plan on my Verizon iPad mini, I used 687MB in the first day.
Now, I'm on day 7. I'm at home on wifi.
I left the iPad on and open last night to an auto refreshing website.
This morning I got the message that I had used up my cellular data...
Something does not seem right...