I wish you could buy a set amount of data that didn't expire in 30 days. If it took you 2 months to use it, so be it. You pay for it and it is yours to use no matter how long it takes.
THIS. If this were possible I'd probably buy a 3G iPad and a few gigs of data. Even if they were expensive, it'd be nice to buy data without having to worry about using it all before some expiration date.
At my typical data usage rates, 2GB of iPad data would probably last me 6-7 months. Almost all my regular iPad use is where I have Wifi.
Of course they won't do this, though, as nearly everyone will pay a lot less, so they'll lose money.
EDIT: I'll note that prepaid wireless carriers "sort of" do this with minutes. You buy a set number of minutes, and typically have a year to use them. There's still a limit (probably to keep accounts from lingering forever if abandoned) but a year is a lot more reasonable than a month. I wish providers would think of data as a consumable, rather than a service.