I had my first one of these today (item lost in transit by the USPS, Tracked Packet USA). I'm making this post in part because eBay has never told us how we're supposed to handle this, so I'm prodding devon@ebay to get us an official answer.
I've refunded the buyer. It went out for delivery about 2 weeks ago, never actually got delivered, then bounced around some sorting facilities. The address is valid according to Canada Post's address complete. I've filed a claim with Canada Post (we'll see how that goes).