That's really not a bad system.  Simply keeping your device offline is a good way to protect it from remote intrusion.  Although don't you need to put that machine online to transfer coin from the account?

I just heard about the bitcoin android app though, makes me think a old android phone with no service plan (so offline) might mike a good mobile wallet.  Of course assuming you have a wifi connection wherever you need to spend anything, you'd also need to use whatever wifi connection they have, and you really can't trust public wifi.