I've got an 07 and mine came "XM Ready". I thought that I could just subscribe but the receiver and some other components were not installed. The antenna was there. I got the instructions from a post here, purchased the missing parts and installed them myself. The dealer quoted me 2 hours to do the installations.... I did it the first time in 45 minutes start to finish including clean up. If I did it 2 more times I'm sure I could do it in 25-30 minutes. Its real easy, you just need to understand how all the different upholstery clips work. They are clearly the most challenging part.. that is: not damaging anything during the process. My advice to anyone with the "XM Ready" configuration, buy the parts and do it yourself. Its really easy.
Actually, everything you'll need is in this post.
http://www.clublexus.com/forums/showthread.php?t=297124