When the SHA-3 winner is announced, and after some time in real use, would it be a good idea to switch to that?
Despite some of the oversimplified solutions earlier in this thread, "switching" to a new hash means creating a new (possibly derived from the existing one) protocol and an entirely new network (possibly based on a genesis block offering BitCoin funds to the SHA-256 addresses that had them outstanding). Back in 2010, there was only a single client, and reinventing everything may have seemed like a simple solution. But beginning with 2011, we are starting to see alternative implementations of BitCoin, and by the time SHA-256 is broken, we will no doubt have many various possibilities. If SHA-3 is due out soon, it might be early enough for all the implementors to agree on reworking the network around it...
Not really true, we can define a future switch block, after which a new set of rules applies. If all developers are notified early enough they can make the switch, and allow time for users to make the switch, when the block arrives old implementations will fork off creating their small network, while the new clients take over the main chain (assuming most of users have made the switch).
No. As long as SHA-256 is used for any blocks in the chain, the entire chain is compromised by a client forging a new block that can sit in-place of the real one in history.