This is a follow up question to "How can I keep my wallet secure?"
Is there a portable (at least Win32&64/Linux) to configure the official bitcoin client to use only offline wallets to be inserted in the usb slot for each transaction?
You can use symbolic links to your bitcoin data folder or wallet.dat to the usb device location. You will still have to run bitcoin -rescan when you start the bitcoin client, and you will have to remove the link if you want to run bitcoin without the usb devices.
Note, however that you do not need to have an offline wallet online at anytime ever unless you are spending the money out of it. If you are only sending money to it then it can remain offline.