Although it's pretty clear at this point, I can confirm from recent 1st hand experience that the residual value can be used by others.

In August, 2006, we had to cancel a flight for my wife, and were left with a credit/voucher. A month or so later, we bought her a new ticket and there was about $180 left over. I kept the original ticket number in a safe place and planned to use it later for my wife. The whole family flew DL last month and I completely forgot about the credit. This month I booked us all some tickets for mid-August. Unfortunately, my wife is only going TPA-ABQ-TPA while the rest of us are going TPA-ABQ-PDX-TPA. Given our flight times and available non-stop flights, southwest makes a whole lot more sense than DL for TPA-ABQ-TPA. So I booked us all on WN from TPA-ABQ and my wife solo returning ABQ-TPA.

I was a little worried that we wouldn't be able to use the credit (and it expires soon), but somewhere in the back of my mind, I remembered something about credits like this being useable by others. I went online to purchase tickets and got to the place to enter e-credits, but I didn't see how to enter a credit from somebody else. So I called SMS and asked if I could use it with them. The agent was incredibly helpful and told me that I can use it online. Indeed, if I had just made one more click I would have found it myself. She said that credits can be used by anyone, "as long as you have the permission of the original ticketholder."

So I purchased the tickets myself online, entering the ticket number and my wife's name, using the credit toward our au pair's ticket ABQ-PDX-TPA.

Just remember that you will need the ticket number and the name of the passenger on the original ticket.

-Michael