Do you mean an area where you CAN'T get wifi, so your only connection to the internet is through 3G on your phone?
Depending on your carrier (and for a fee), you could set up your iPhone as a wifi hotspot. You can also just outright buy a wifi hotspot (with monthly fee) from many carriers. Then, there's probably a jailbreak option (which would be free), but I have no info on that.
Regards,
Tom