Well, I think the reason that things expire is so that he can then reuse those words (since there are only so many short words that anyone can spell), but that is a good idea for ones that have expired but now yet been reallocated.
Might be it could be expanded to a combination of two words, like amazon does for basket checkout.
If the key has been reallocated, it is no longer expired.
The link you created is. If you create a link mapping foo -> google.com, and then it expires and someone else wants a link to facebook.com and shoutcast picks foo as their word, as far as you're concerned, your link is expired because shoutcast.com/foo doesn't get you to Google anymore.
Yes, from the submitter's perspective, their link has been reclaimed. From the service's perspective, a key is either in use or not, and we agreed that keys that aren't in active use could have ads on them.