Multi-Column Constraints and Null Values

[Phillip Tornroth <ptornroth(at)intellidot(dot)net>]

I have a problem in a number of my tables. I'd like to add uniqueness  
constraints across multiple columns, but some of the columns are  
nullable. This gives me trouble since when I create a constraint on  
columns A and B.. I'd like the constraint to be enforced such that  
you couldn't insert values ("value for A", null) twice. I understand  
why the constraints work this way, but I'm wondering if anyone knows  
of a workaround.

Feel free to spare me any "don't use nullable columns" responses. I  
of course am aware that is an option. It's just one I'm hoping to  
avoid as this is a legacy database and it would be a mammoth effort  
to accomplish.

Thank you in advance!

Phill