I would like to give a +1 to the USB key approach already suggested in a couple of the prior posts.

Whenever a new wallet.dat is created the client forces the creation of a USB key that must be plugged in whenever bitcoins are to be sent to another wallet.

You could use a 2GB SD card with a USB adapter for this because its cheap and has the added advantage of a write protect switch.

There is no reason I can think of that one SD card can't be used with multiple wallet.dat files and you should be able to copy one SD card to another for backup purposes.

To SEND any coins you have to enter your password AND plug the SD card into a USB port.

Bitcoin is a project in Beta, exactly the reason to test the system in the real world and arrive at the best possible solution. It is slightly in danger of falling victim to its own success.