Amusing you should mention DB2. Yeah, they're the database we're trying to beat in terms of predicate locking.
The developer who wrote it, Kevin Grittner, is implementing some academic papers on how to do serialization and predicate locking which haven't previously been implemented. Part of that is already committed, so pick up the next Alpha at the end of this commitfest (around October 17th) and give it a spin. I know Kevin would love some feedback.