Are you guys referring to the cabins at Phantom Ranch, or to camping at Bright Angel Campground (in the same area as Phantom Ranch)?
I got a permit to stay in the campground fairly easily with some advance planning. It does fill up very quickly, but if you make sure to get your permit application in early on the first day you can, and also have backup dates you should get something.
I don't have any experience with reserving cabins at Phantom Ranch proper, so can't help you there. I do hear it gets booked well in advance though.
