How do I recover Bitcoins from a Change address that is in a corrupted wallet? I am running Bitcoin-Qt for Mac 0.8.1 beta 

I received a "wallet.dat corrupt salvage failed" error and the client refused to launch.

I replaced the wallet.dat with a backup made a few hours earlier and on launching the client, I found that almost all my Bitcoins had been transferred to an alien address which I did not recognise, and had certainly not initiated.

One possible explanation is that these Bitcoins have been transferred to a Change address. But how can I access this?
If that really was a change address, its private key is included in your backup of the wallet, even if you can't see it in the client GUI; this is because Bitcoin-QT always keeps some (a hundred, IIRC) precomputed addresses ready, and uses them whenever you request a new address or it needs a change one. However, change addresses are never shown in your address list, so you couldn't see them even if this was your original wallet.

If those Bitcoins have really been sent to a change address, you'll still see them in your balance, and you'll be able to spend them. However, if your balance is actually less than before, then they'we been transferred somewhere else, thus the trojan hypothesis sadly applies.