Complex constraint across all data in a table

Question

We have a table to record processing that occurs on a system and we need to ensure that only a single row has an 'in process' status.

I want to ensure that the result of this is always either zero or one:

select count(id) from jobs where status in ('P', 'G');

We're using explicit transactions so ideally this check would happen on commit and abort the transaction if the invariant doesn't hold. Handling any racy processing that throws the occasional error is much easier for us to deal with than suddenly ending up with more than one job that is 'in process'.

The solution only needs to work with Postgres so we're quite happy to take a non-standard solution to this. We're currently on 8.4, but we'll be upgrading at some point to 9.x if that makes any difference.