I think we're going to hit an (arbitrary, psychological) tipping point when the price of 1 BTC equals the price of 1 (troy) ounce of gold - currently around 1,500-1,600 USD.

When that moment comes, I think a certain percentage of people who invest in gold will suddenly "see" that BTC shares many similar useful properties, but BTC has much better "versions" of these properties (eg, you can put multiple copies of your wallet on pendrives, paper, brainwallet; you can take it on a plane or encrypt it and email it to yourself, etc.)

So I envision a certain percentage of gold holders switching some of their holdings to bitcoin.