From my most recent billing statement (October 2-November 1) I was charged an additional $10 for "Unbilled usage from previous months". I have a 2 GB plan for each phone, I upgraded from Samsung Galaxy on unlimited data plan to the iphone 5 on September 21. My data usage for the month was 0.20 GB (rounded up to 1 GB), my "unbilled usage from previous months" was 1 GB (I assume also rounded), which totals 2 GB - I did not exceed my 2 GB limit, however I was charged an additional $10. Please see screen shot below:
