I did mine without taking the seat out and didnít have any problems.
I used the replacement pads that I bought from an online GM dealer.
There is a picture in this post of the seat bottom from when I did it: http://www.gmforum.com/t270249/
The butt leather came up fairly easy- just push down & unsnap the plastic around bottom edges and separate the Velcro in center (remove bezel around seat control buttons first).
From what I remember about the back leather, I unzipped the back and I think I worked it up from the bottom just enough to do the job. I didnít mess with the airbag (you can disable it if you are concerned).