Got it from Pocketgamer:
As long as you own a device that can create wi-fi hotspots, (like the iPhone 4S, for example), you're good to go on this one. Why this function is hidden is pretty obvious to all - Sony would like you to buy the Wi-Fi + 3G Vita model - but you can save yourself a pretty penny by just connecting to another device with access to the net.
All you need to do is ensure you've got the personal hotspot option on your smartphone set to 'on', then connect to it from the Vita as if it were a wireless access point. 12 words of caution, though: the Vita doesn't like the way Apple handles apostrophes in device names.
So, make sure it's set to the grammatically incorrect "Peters iPhone" instead of "Peter's iPhone", for instance. Might infuriate some people, but it's worth it for up-to-date MotorStorm RC times anywhere you go.