Aluminum was considered a precious metal in the 1700 and 1800s. It was very desireable, and very hard to obtain with old mining methods.

Anyway, I don't see Bitcoin replacing precious metals, as they are completely different. First, Bitcoins are still very new technology. They are an incredibly risky investment, because the chances of failure are still there. Precious metals have been used for 5,000 years. People worry about the price, but they don't worry about their power going out and being completely unconnected to their stores of wealth and finances. There will always be that draw to tangible assets, which also explains the Casascius Coin craze. If someone was to ask me right now what is a better investment for 20 years from now, Bitcoins or Gold/Silver, I'd probably say Gold/Silver.