I've started work on a memory mapped soprano RDF backend. Given that mobile devices use flash for their permanent storage, an RDF backend designed using primary storage algorithms should work well on maemo.
While the SPARQL implementation is "far from complete", it can already evaluate some common queries very very quickly. I have some triple matching and the ability to have multiple regex filter statements. Other filters and more complete SPARQL lanugage support should follow in time, as time permits... patches accepted etc and so on.
For those interested, see soprano-boostmmapbackend on my main sf.net page.