I see no reason to anticipate Bitcoin moving to a new hashing algo, given the investment in existing hardware, and that the hardware owners are the ones who vote with their gigahashes on how the network functions.

However, even if it were to change, there are several altcoins that use the same hashing algorithm (PeerCoin being the biggest), and/or someone will invent a new blockchain service to take advantage of the existing hardware. While I think there is great merit to ASIC-resistant hashing algorithms of ProtoShares/PrimeCoin/etc, that ASICs would ever become useless seems highly unlikely.