Well ... it doesn't want to be zero, that's for sure. This sounds like a wart left over from the CMS->ACS integration effort.
I think NULL's OK and PG 7.3 supports the removal of this constraint, doesn't it? And we're requiring 7.3 for 5.0 and since I assume your change is for 5.1 (since 5.0's a freeze) then you'll be able to generate an upgrade script.
Note that this is a TIP item ... you don't just get to do it yourself, it's a fairly significant change.
