When you imported the paper wallet address into the client, you made the coins in that address available to the client, but you didn’t make a transaction sending coins from that address.
This is why the balance didn’t change.
I have a paper wallet with 100 PPC. Once a week, I check the 100 PPC is still “there”
However, from what you are saying, if a hacker compromised the paper wallet, found out the private key, and imported the 100 PPC into his own client, my paper public address would still read 100 PPC
So, how is it possible to tell whether a paper wallet is safe and intact?
When you know the private key to a public address and you can see that the coins still are at that public address, you still control those coins. An attacker will take that control away from you by moving those coins to a public address that only he knows the private key for.