is a unique key on null field bad?

So, we are trying to track down some problems we're having with an
implementation of slony on our database.  I've posted to the slony list
about this issue, but I wanted to get a more generic response from the
perspective of postgresql.

Is it a 'bad thing' to have a unique key on a field that is often times
null?  This application has been running along just fine for a couple of
years now, but when we try to implement a slony replication solution,
this one table consistently has inconsistent data between the primary
node and the slave.

The problem we are having with slony seems to be related to a table that
has just such a key, so we are trying to figure out if this is causing
the problem.

--
Until later, Geoffrey

Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety.
  - Benjamin Franklin