> I think unit test and assertions (in the form
> of precondition and postconditions) can both be used
> at the same time, depending on which is best suited
> for the case being. So it's not an absolute choice :-).
> Also, as Markus has started to build test-specific
> refactorings, I think that implementing "move to assertion"
> and "move to unit test" refactorings is possible and useful.

Yeap we (roel and I) wanted to see if the tests can be automatically 
refactored when the code is  refactored.
So may be one day...