>I [assign a PK] in the Record Validation Rule; if the key field is empty, I determine an appropriate value for the key and REPLACE the key with it.

Interesting. What would you say is the benefit to doing it then, instead of making the key determination the default value?

A drawback that I can see is that you don't find out what your PK is until TableUpdate(), and I have one app where I add both parent and child records at the same time. The child needs to know the parent's PK right away.