Here is what I have tried so far:
I entered the mini private key S4b3N3oGqDqR5jNuxEvDwf and got the response "Private address added to your virtual wallet with address 1GAehh7TsJAHuUAeKZcXf5CnwuGuGgyX2S. There was no coins in there, maybe you did a typo?" This is the correct public key address and there are no coins at this address. So far so good.
I entered the hex private key for the same key 0c28fca386c7a227600b2fe50b7cae11ec86d3bf1fbe471be89827e19d72aa1d and got the response "Your private address could not be redeemed (already redeemed?)". I guess this is the expected response and it proves that they calculated the same account for this version of the private key.
However, when I enter the WIF 5HueCGU8rMjxEXxiPuD5BDku4MkFqeZyd4dZ1jvhTVqvbTLvyTJ for the same private key it calculated the wrong public key address. So there appears to be a problem/bug when using the WIF.
I reported this bug to Mt. Gox.
I then used both a real mini private key and a real hex private key to attempt to import Bitcoins from active accounts (opened physical coins). In both cases it calculated the correct public key address and found that there were Bitcoins in the accounts and promised to import them into Mt. Gox.
The Bitcoins were eventually moved off the accounts for the private keys I entered into Mt. Gox and placed on new accounts withing Mt. Gox. This proves that they sweep the value off the imported private keys so there is no way to reuse the private key to send Bitcoins out of Mt. Gox.
See
http://blockexplorer.com/address/13MsUf2Nnowoj8gdZBfBSyBJzVcC1R7hZAAnd
http://blockexplorer.com/address/13MT4QvExmajXAYqKQeMXAh9saVTVP6f6aBoth (one from a mini key and one from the hex key) showed up in my Mt. Gox account: The notation looked like this:
Fri 07 Oct 2011 02:30:00 PM GMT Deposit 18kvvRuTdb5SpwcGtmvaBJMCCrb7hHcbXu 13MsUf2N mini 1.00000000 BTC
Fri 07 Oct 2011 02:29:01 PM GMT Deposit 1Mcdeft9J9raPEgdMHTw97UUoS9c8xyoqN 13MT4QvE hex 1.00000000 BTC
This shows both the notes I put in when I imported the key (first bits of the original public address and the private key type used) and the new public address the Bitcoins were swept into.