:: is not a value, so there is no way to know whether a null is
:: unique of not. (Null means "unknown"). It was relaxed in

I guess this depends on your philosophy - Ive always treated null as "empty"
rather than unknown.

:: mechanism - "I don't care if the value is unknown, but if
:: it's known, then it has to be unique".

Yes, which is how most other DBs treat it. Im really quite surprised to hear
that pre 1.5 it was otherwise.