Semantic stuff (RDF, OWL) on mobile phones - is it possible?

You wrote you want J2ME, but other readers might be interested in C#.

Mono makes C# available on iPhone and Android. Once that is done, you can use ROWLEX to deal with RDF and OWL. You might consider reading this Stackoverflow question.

Maybe look into IYOUIT. It is a

mobile application developed in Python, and running on Nokia Series 60 phones.

It uses OWL and reasoning. You can read the details in this paper.