The first release candidate for open source PostgreSQL 9.0 is now available.
“Please download and test immediately so that we can move rapidly towards final release. All known bugs should be fixed, so users should promptly report any bugs which they find,” the announcement said.
An initdb and database reload is required because of a system catalog change when upgrading from 9.0 beta versions.
Release notes are available here and the source code can be found here.