Does this mean that I don't have to use the same machine?
No, why? The wallet is just that, a wallet. It contains the private key. If you put the wallet.dat in the roaming/bitcoin folder and then you start bitcoin-qt, it will read that wallet and let you use it.
Having Bitcoin-QT running on multiple machines, can I take the saved wallet.dat and open it with ANY machine running the same version of Bitcoin-QT?
Of course yes
And if it wasn't encrypted, could that burglar with a copy of wallet.dat open it on any machine with Bitcoin-QT and shoot the money over to silkroad or something?
Yes
The wallet.dat file contains the private keys, if they aren't encrypted and someone steal them, of course they will have them and will be able to do what they want with your bitcoins