Here's the address I used:
Norman Reedus
"The Walking Dead - Season 3"
Raleigh Studios Atlanta
600 Chestlehurst Rd.
Senoia, GA 30276
I sent him a Boondock Saints poster to autograph for a good friend of mine for her birthday. I wrote him a letter explaining how we were both fans, and how much I appreciated him taking the time out of his schedule to read the letter and to autograph the poster. I sent the poster in the original shipping tube it was in when I ordered it online with return postage and a shipping label to my address, and had it packaged in a priority mail box big enough to accommodate it. It took just over two weeks to get the poster back. My only complaint was that when USPS dropped off the poster tube, it had been crushed in the middle, so the poster wound up with a few creases in it, but hopefully they'll even out eventually.
Special thanks to sfgiantsfan for the advice on how to go about writing Norman, and thanks to this forum for the up to date contact information.
Sorry for the poor quality of the pictures, they were taken with my cell phone:

