Could someone do this? Yes.
What are the chances of this happening to you? I would say small. I have just received identity theft training from a state detective who specializes in it. Most of the thefts occurring in the past few years have been relatively simple compared to wifi hijacking. For example, in rural areas that still have roadside mailboxes, people just walk right up and get your mail out. Even more scary, there was a case in kentucky where someone paid a bank teller for cutomers' info. From my point of view, you don't stop using a bank because of this. People don't quit their jobs because their empolyer gave all their personal data to the UPS guy. So, I would say you're relatively safe on wifi. I think the main thing to do is be as careful as possible and MONITOR, MONITOR, MONITOR your personal data. It is not just the credit bureaus that now have to give you a free report on yourself - it is other data collection agencies as well (Choicepoint ring any bells?).
Also - you can put a fraud alert on your credit card, which means that lenders are not supposed to grant "instant credit" in your name (such as stores, etc.) If you have not been a victim of identity theft already, I think you have to call and have this renewed every 90 days. Not convenient, but since most fraud is simple credit card fraud at this point, it could save you lots of headaches in the future.