J Moore , Apr 24, 2005; 11:19 p.m.
My wedding photographer did this for us (it is something he does for all of his clients). We have it hanging above our sofa in the living room. I'm pretty sure he just used a standard mat like you'd get at Aaron Brothers (or any other frame shop) and an extra fine point Sharpee (probably black would work best). The mat, for obvious reasons, has to be a lighter color (ours was a light tan color). Since we had a small wedding (24 people including us) it was wonderful because all of our loved ones had room to write us a note, but I've seen it at huge weddings too.