Just had a question about 3G data on the iPad. I only own a wifi version. I know it's 25 dollars for 2 gigs of data. Is that a month? Or does the data carry over? Such as can you pay 25 for 2 gigs and make it last for 6 months? I use my iPad mostly at home on wifi but who knows when I'll need 3G. So just curious on how it worked. Thanks