K.K.
we also used Viacord, not cheap but great service -- I have one friend who called to set it up and then went into labor that night, and they couriered the collection stuff to her at the hospital in time for the birth (usually you want to give the process several weeks if you can....). they make it pretty easy to get it done.
honestly, it just feels like a bit of an insurance policy for your kids' health. and while there are no guarantees, the likelihood of a match for siblings is pretty high, too, so for us, when we found out that we were having identical twins as kids #2 and 3, it seemed like a no-brainer to at least bank one kid's blood.
as for the poster who said that you should wait before clamping the cord, we did. you can wait several minutes and still have enough blood for collection. I'm all about natural birth and letting the cord stop pulsing and immediate skin contact and nursing, and I still say, let science do its thing when it can without interfering! :)
good luck with #2!
K.