Seems to work fine on my end. Are you sure you're tossing in the proper get variables for your script to return anything? I'm guessing that you need an ampersand somewhere in there:
Then again, I might be wrong, but... you do know that jQuery get can also send the required variables on it's own too, right? You could always try the following:
See how that works for you. Additionally, consider getting FireBug and seeing exactly what the php script throws back your way.