insert locking issue for PG 9.0

Our application has a table that looks like:

create table jobs
(
    id int,
    first boolean
);


What we need is for the app to be able to shove data into jobs with an assigned id, and guarantee that first is only
truefor one id. In other words, we could easily enforce what we want by creating a unique index on jobs (id) where
first=true.

The problem comes in how we deal with exceptions (and there will be many). We'd like to just have the database say,
"oh,hey, there's already a row with this id; I guess I'll make first=false". If we were to wrap inserts to this table
intoa stored proc it seems like that would be easy enough to handle those exceptions and try to re-insert with
first=false,except that this is Rails and calling a stored proc instead of doing inserts will be difficult in this
case.So that's pretty much out, if we can avoid it. 

We could use rules to call that procedure INSTEAD OF inserts. That seems like it should work, but.... rules. Also, it
requiresus to keep an index that we don't need for anything else. 

It would be nice if there was a way to have a before trigger function on jobs that would twiddle first as needed, but
we'vebeen unable to find a way that doesn't involve a race condition or lock escalation deadlocks. Advisory locks are
releasedbefore the new row is visible, and "normal" locks stronger than what INSERT acquires leads to lock escalation
deadlocks.

We've considered using rules to acquire the strict lock, then a before trigger to do the twiddling, but then we're back
tousing rules. 

Does anybody have any elegant ideas to make this happen?