If you can donate your cord blood, please do. My little sister (now 13) is alive today because of bone marrow transplant from an anonymous donor. If, for some reason, your family needs the cord blood, later, you can use it, as long as it hasn't been already used. You can find out how to donate it here:
Cord Blood Donation: Frequently Asked Questions
It takes a little while to do the paperwork, but if you can, please do. You never know whose life you could change. If you want more info about my sister, you can PM me. :)
Oh, and it's true that often the cord blood cannot be used for the same child (however, in some cases it can, including leukemia, depending on the type of treatment it's being used for), but it can often be used for a sibling. My other sisters and I were not a match for our sister, though, so it can't always be used.
eta: We had no family history of the disease my sister developed, and most children don't! Just wanted to add that.