I put my name, the date, and sometimes the town/state I live in. If it is for a special occasion, I put that on it too (eg. "For Grandma, Christmas 2009"). That is not so much for posterity as it is to make the gift feel more personal to the recipient. Most of the time the label is only about 2 inches by 3 inches, but some have been much larger (a group quilt given to a lady at church had to have ALL our names on it), and once I used a spare block and wrote the info in the background spaces.
For charity quilts, I put a label with the date and a "This Quilt Belongs To" spot for the child's name.
You can never tell what will make it through the years and be passed on, and I wish there were labels on a couple of the quilts I recieved passed down from family. As it is, all anyone has is a vague idea of who COULD have made them.