Great idea - that's what quilters did many, many years ago...they used old clothing or scraps from making new clothing. Fabric for quilting is a "new trend".
Check out Quiltville.com for lots of idea on quilting with scraps and old clothes. Bonnie says find a pattern/quilt you like, see what size the pieces are cut down to and cut your shirts to that size. That's what I'm doing with my shirt collection.
I've seen a quilt that incorporated the necklines/tags right into the quilt...it was cool. They included pockets (some with the flap and button still attached.