It doesn't have to be called "wallet.dat", right? That would be too obvious. Can you call it anything, with any extension?
Bitcoin-Qt is just not so trivial to me to use since the trivial way would be to have an option File\Open wallet (at starting the program), then enter password if encrypted. TrueCrypt for example is trivial in this way, you select any volume, type in password, mount, dismount images.
I don't understand that either that you encrypt your wallet.dat file then close client, open client and doesn't ask for your password?