I have a local Bitcoin-QT wallet. Can transaction malleability be used to steal my horde of tens of thousands of Satoshis?
No. It can not under any circumstances be used to steal funds from a Bitcoin-QT wallet.
Transaction malleability has a very small effect on a regular user, at most it can mess up some of your bookkeeping if you store the Transaction Id of a transaction before it is confirmed.
Transaction malleability only affects unconfirmed transactions. The bug allows an unconfirmed transaction to be duplicated with a new transaction id. Since the two (or more) transactions are spending the same input only one can go through, but that one of them should deliver the money just as expected. For major services that rely on a big number of unconfirmed transactions, this can cause some headache, but there is no way it could be used to steal money from your Bitcoin-Qt wallet.