Unfortunately, there is no centralised place. It should probably be a CVS branch but you’ll need to pester a core developer for that. My various hacks at getting wordpress to use PostgreSQL have royally hosed the ability to use MySQL. If the developers would stick to reasonably standard SQL statements, the port would be trivial… I’ve more or less given up on getting the current version of wordpress working with PostgreSQL. I figure it would take about 1-2 weeks (f/t) to get the code working with PG again, and probably another to get it working with both. For me, my ancient, ‘alpha code’ works well enough.