The critical bit for you is the kernel version number. Petespin27 is right, Airports use a Broadcom chipset, and until the bcm43xx was put into the kernel in version 2.6.17, PPC users were pretty much out of luck. So as long as Ubuntu 7.04 has a kernel that is 2.6.17 or later, you should be good to go. You probably also have to install firmware, and there is a crude how-to
here.