Are you planning on getting the autographs and then making a quilt or making a quilt that everyone signs?
I have done it the first way -- I liked having the control of giving a "bad" autograph a new piece of fabric. Mine was actually an autograph/draw-a-picture quilt for our Kindergarten teacher. I prewashed the white fabric, cut it in 6 inch squares, and then gave one to each child with a fabric marker. After they returned them I ironed the squares to "set" the ink and sewed it together with fun fabric.
If you pre-cut fabic you might want to use a washable quilt marker to indicate where not to write, you know your 1/4" seam area. Also, make sure you use fabric markers and not permanent markers. The permanent tends to bleed.
Is this for a wedding, anniversary, or graduation? You could also include a few photos in the quilt. They sell fabric that you can put through your printer.
If I were doing a quilt that was going to be signed after it was made, I would think of how many sigs it would be getting. If you are thinking a smaller number (say 25) then I might do a more traditional quilt where there was the right number of spots available for signing. Like a flower quilt where the centers of the flowers get signed. If it is a larger number, then make a quilt with a wide border and have them sign there. Or big squares in a pattern.
You can probably tell I get a little anal and want to control where they sign! If I had someone signing a pre-made quilt I would make sure I had someone with the quilt at all times so a kid didn't just pick up a pen and scribble.
Sounds like fun - please let us know what you are going to do!