New features include: Improve concurrent access to the shared buffer cache. Allow index scans to use an intermediate in-memory bitmap. Add two-phase commit. Create a new role system that replaces users and groups. Automatically use indexes for MIN() and MAX(). Move /contrib/pg_autovacuum into the main server. Add shared row level locks using SELECT...FOR SHARE. Add dependencies on shared objects, specifically roles. Improve performance for partitioned tables.
PostgreSQL 8.1 has been released. You can download from here, and the source code from here. The release notes are here.