I have a quixotic project I've been tilting at off and on for 5 years now, where all the existing solutions make poor use of relational databases (usually SQLite), but the problem maps directly onto an EAV graph database. I was worried I'd have to write my own query metalanguage, but now I don't have to!
It's depressing how much the W3C salted the earth with the way they tied RDF to the twin loadstones of terrible XML tree serialization and SemWeb wankery. I couldn't find any way to get anything useful done with their SPARQL either.
EAV graph databases might have their day in the sun thanks to the NoSQL hype-wave. AppEngine's BigTable-backed datastore is a terrific exemplar, hopefully other players can come up with similarly compelling APIs and hosting stories.