If you have owned an iPhone, chances are you hate AT&T. If you don't have AT&T, then you would probably rather use your PlayStation Vita with whatever carrier you already signed your life away to.
Unfortunately, the PlayStation Vita is carrier-locked to AT&T. Swapping the Vita's SIM card to a different carrier's won't do diddly.
This sucks, yes. But I wouldn't be surprised that after some time of exclusivity, Sony will reach agreements with other carriers down the line as the Vita install base grows. Just as Apple has with the iPhone and iPad.
Until then, you're stuck with AT&T and their somewhat reasonable 250MB for $14.99/month and 2GB for $24.99/month plans.