Aaaa, so it decyphers only in RAM when im doing transaction? It is kept cyphered on the disk all the time, so the lock is unneeded?

On Windows XP i've set up Bitcoin-QT 0.8, synched with network, encrypted the wallet.dat file.
I have status stating that the wallet is encrypted.
Now decided to make security test.
I copied the wallet.dat to pendrive while Bitcoin was run - and here first bad surprise. Why isn't it really locked???
Than i installed Bitcoin-QT 0.8 on another computer with Windows 7. Only instead of creating wallet i copied one from pendrive.
And another bad surprise. The wallet was opened without asking about passphrase and i has all access to my copied wallet!!!

So to me any child can write a virus that will copy and send to himself wallet.dat because the lock doesn't work AND/OR wallet is not really ciphered, than transfer all my bitcoins to himself.

I feel like i discovered America. WTF? Is there ANY security in Bitcoin, or i miss some point?