I too am having the same problem with callback.
I have the latest copy of Zencart and the latest copy of WorldPay module. (WorldPay_ZC1.3.x_v2.01)
I do get the Callback Failure Alert From WorldPay (302)
I have enabled the debugging email.
I have check that it is sending the correct password through and this is where I have got stuck.
There is no mention of the word callbackPW or the password its self in the debug email?

Can any one help me?

Paul J Thomas