Apache is participating the Google summer of code (GSoC) and Jena's
taking part.
If you might be interested and eligible, find out what it's all about
[0] then join the dev@ mailing list and introduce yourself.
Projects ideas can be whatever you are interested in working on or check
out JIRA and look for things that you want to do [1]. There's a
gsoc2013 label but these are just suggestions. When you introduce
yourself, you can describe what your interested if you want suggestions
from the developers.
[0] http://community.apache.org/gsoc.html
https://google-melange.appspot.com/gsoc/homepage/google/gsoc2013
[1] http://s.apache.org/jena-gsoc2013
Andy