Every private key I entered came up with the response that the wallet was empty and it might be a typo. Besides that message it seemed to have accepted the private key because if I tried to resubmit it would tell me it was already uploaded.
Out of curiosity, try sending a reasonable amount (e.g., 0.01 BTC or above) to an address that you have already imported to your Mt. Gox wallet. If that gets swept but the "dust" doesn't then you have your answer.
[Update:
From another thread:
18:02.47 ( @MagicalTux ) must have been some changes with the new version of bitcoin
18:04.19 ( @MagicalTux ) we have a project that will solve that, I guess I'll have to push it sooner than expected
18:04.59 ( kakobrekla ) any eta on when these get credited?
18:05.57 ( @MagicalTux ) I guess we have created the txs, but it's stuck in the transfer queue because bitcoind randomly refuses connections
]