What is the NTX ID that Mt. Gox refer to in their Feb 17th Statement?
This is closely tied to the transaction-malleability bug, which allowed the re-posting of unconfirmed transactions with an altered transaction id. The issue increased the complexity for bookkeeping for services with a high number of transactions. Especially, some people used it to sneak duplicate withdrawals from Mt.Gox.

The change to the transaction ID was made by pushing additional zeros to the signature's front. I think NTX ID stands for normalized transaction id which will map all duplicate transactions back to the same representation, even when they were altered by exploiting the transaction malleability.

It is an internal blockchain representation used by Mt.Gox and has nothing to do with the service blockchain.info.