MtGox will sweep any coins out of the address when it is added, and will keep it for future use and sweep out any coins added later. The coins are sent to a new address and credited to your account.
So if I import a private key, I can use the address forever, and whenever someone sends money to it, it goes into my Mt. Gox account automatically. Right?
If you import it to MtGox, I would not recommend that you use it in your personal wallet. However, yes that is how it works. You may wish to send a satoshi to the address first before importing it, because I'm not sure whether MtGox will import an empty key.