"is null" vs "== null"

[Unknown W. Brackets]

I have a silly question.

When is comparing against == with opEquals *not* an error?  I mean, when 
should that ever pass if opEquals is available to be called?

I can see a class that says, "I'm null currently" but that seems about 
as logical as a class that overloads operators to do bizarre things.

-[Unknown]