tool for comparing databases (?)

Does there exist a method or tool to compare two PostgreSQL databases?

Say you have an application based on PostgreSQL and it is deployed to
dozens or hundreds of disparate systems. Different versions (v1.2, v1.3)
of the application require somewhat different database schemas (e.g. v1.3
has an extra column on one table). Over a long period of time and many
upgrades, it would be worthwhile to validate that your database fits the
expected schema. How could one acheive this?

This seems like a useful general purpose tool, so perhaps this question
has already been addressed.

If one could "pg_dump" the system catalogs, they would have database
schema snapshots. Next, they would need a tool to compare two snapshots
(in whole or in part) and determine the relationship (A matches B exactly,
A contains all of B plus + some extra X, or A contains all of B except for
missing  Y).

Comments? Suggestions? Interest?

Mike Adler