At the end of the day, only your machine will be able to access the wallet file. using the file on another machine will not work, even with the application. This will cause a problem if your machine was to fail, or if you planned to use the wallet on more than one machine. But there will eventually be measures to protect against this too.
Which is still possible to steal. If the attacker has remote access to the machine via a trojan he can still steal coins in a variety of ways.
1) simply use wallet to transfer them out to an address he owns.
2) grab decrypted keys from memory.
3) decompile the client to determine what hardware specific strings are used to generate the decryption key and decrypt the wallet file.
Tip for next time. You might get more interest without stupid claims like "impossible to steal".
You keep using this word "impossible". This word, I don't think this word means what you think it means.