There are no interference issues, but there are potential security issues. A GPS unit can certainly pre-programmed with the position of a potential "target" and allow a hijacker to fly right to that target. This ability is obviously only possible if cockpit security is breached.

In fact, I have argued that the 9/11 hijackers must have used GPS guidance to hijack planes hundreds of miles from their intended targets and still find them with their very basic piloting skills. I don't care how clear the weather was that day... it would be very difficult to take over a 757 at 35,000 feet over Ohio and fly a virtual straight line to hit the Pentagon without GPS guidance. However, many people disagree with me on this.

A thread from last summer:

http://www.flyertalk.com/forum/showt...t=gps+security