It doesn't (as far as I'm aware) check for collisions, and (again, as far as I'm aware) its granularity is at the table level, not individual columns, but you may want to look at Slony-I anyway. It's a PostgreSQL-specific 'single master, multiple slaves' replication solution and may get you at least some of the way to what you're after.

There also DBD::Recall, but it's not been updated in a while, and the documentation states:

DBD::Recall is a hack that attempts to accomplish something (fault-tolerance through replication) at the perl DBD driver level that would be better implemented by database servers. It works, but it is not pretty.

Hope that helps