From what I had originally heard, you have someone from CGC witness the signing. Now what I discovered at this years NY Comic Con is that in order to get your book Signature Series, you have to actually give your book to CGC and have them get it signed for you and then they mail it back to you.
Now it very much ups the value of your book but that's bullshit. As much as I'm trying to make a buck here I'm very much a fan of everything I'm trying to sell. I have every issue of The Walking Dead from issue #36 on and all the trades prior to that. I have the first 4 hardcovers of Invincible and Volume 1 of Saga. When I meet these creators, I idolize them and am honored to meet them. I get pleasure out of having them sign my books and to have someone else take that away ruins all the pleasure out of the autograph in the first place.
Also, I'm the one who has the proof that it was autographed. I don't trust them to take my stuff and mark it up without me being there to witness it.
Below are all of my CGC Graded books. I witnessed the creators autograph them with my own two eyes. They're Signature Series in my book.