Can wallets be shared by different machines? I have a bitcoin client installed on machine one. Let's say it has 5 coins in there. I then have another machine, with another client on it. Can both machines see the same wallet?

Do I have to have a per machine wallet? What if I want to pool my coins in one place?
It is a bad idea to have multiple copies of your wallet.dat running from multiple computers if both will be spending.  You risk getting double spend attempts that will never confirm and cannot be cleared without performing wallet surgery.

BlockChain.info/wallet has a "desktop sync" feature that will let their hybrid EWallet sync with your Bitcoin.org client (through RPC), but since BlockChain.info/wallet can be used on multiple machines why not just use BlockChain.info/wallet from everywhere instead?  You get to use the mobile app with it as well.