multi-column unique constraints with nullable columns

I have many tables who's natural key includes a nullable column. In this cases it's a soft-delete or 'deprecated' date time. I'd like to add a table constraint enforcing this constraint without writing a custom procedure, but I've found that postgres treats NULLs very consistently with respect to the NULL != NULL behavior. As a result, when I define a constraint on the last two columns in these insert statements... they both succeed.

insert into mytable values (1,300, null);
insert into mytable values (1,300, null);

This is frustrating, and while there may be someone who actually wants constraints to work this way... I can't understand why.

Now, I understand that the best way to solve my problem would be to use only non-nullable columns for my natural keys. I actually plan to do that, and use a very high value for my 'undeprecated' date to solve most of my problems related to this. However, I can't release that version of software carelessly and I need to tighten up customer databases in the meantime.

Is there a way to get the behavior I want?

Also, is this in compliance with SQL92? I'm surprised constraints work this way.

Thank you,
Phill