James, tyrtell it's just not a good idea. like 750r said they both run on the same frequency range so interference is a strong possibility. Yes, it can be done and it's not guaranteed your Phantom will crash (I myself have accidentally taken off before with the gopro's wifi on..realized it and returned), especially at short range but the farther you get away from the Remote Control, the weaker that signal gets and there will come a point where the (wifi) broadcast from the GoPro right there next to the Phantom is stronger than the signal the Phantom is receiving from the RC... and that's when the problem happens. Maybe it'll come home... maybe it'll autoland... maybe you'll be able to get control back, maybe not. maybe it'll fly away never to be seen again.
the point is, it's just not worth the risk. why risk your $1000+ phantom flying off into neverneverland for shitty, laggy super short range "fpv" using the gopro's wifi when for a few hundred more dollars you can add a powerful 5.8Ghz FPV system that won't interfere, isn't laggy, and reaches as far or farther than the Phantom's range? That's what the (non-Vision) series Phantoms were designed for... NOT to use the gopro's wifi as clearly stated in all the documentation.