Did someone say, Alpaca Socks?
I went to the alpaca sock website linked to from the Bitcoin wiki's alpaca page and here is what it said:
“Each pair of socks is 2.5 BTC delivered to your door, no extra fees for shipping within the U.S. This price may fluctuate in the future depending on the current BTC exchange rate.”
The fact that the “price may fluctuate” means bitcoins are not backed by alpaca socks. There is not a promise but only a temporary promise to exchange alpaca socks for 2.5 BTC and that temporary promise can change at any time.