I have no doubt they already have an abstracted data access layer, the problem is that MySQL and PgSQL both support slightly different versions of the SQL standard plus their own non-standard syntax. Supporting both is doable of course, just not trivial, especially if you want to optimise for both.
I am sick and tired of MySQL - especially now that Oracle has made their disdain for open source and obviously by extension MySQL so apparent with this lawsuit against Google over Android(but really it's over the Java core). It's also becoming clear as day that there will NOT be a viable, interchangable alternative to MySQL probably ever. If MySQL ever had any shot at having a scaling instance like Oracle does, it's definitely dashed. It is on a road to a sad, quiet end. Looking to the future, if I can't replace MySQL I'll just be forced to replace vBulletin. And to the people talking about proprietary operations or encoding methods, it should be noted that Postgre supports the full SQL standard, whereas MySQL is the proprietary one.
Also can we get a really good migration process from VB to Xen ~
I remember on day one there was word about consideration in the future for additional database layers, but I can't find the post. That said, I do remember clearly they said at the moment they will focus on only one, MySQLd.