Although we don't officially support PostgreSQL 9.0, I've been using it locally for development. The only problem I've hit so far is that installing a plugin into Confluence fails with an unknown error in the UI.

An unexpected error occurred

This is because the JDBC drivers we ship with Confluence have a problem with the BLOB data in PostgreSQL 9.0. To work around the problem, you can download the latest JDBC drivers.

We should update the version of the drivers we ship in Confluence to 9.0-801.jdbc3, which is already available in the central Maven repository.