At the moment I'm trying to compile the meego/N9 contact & calendaring code on my ubuntu box. Because really I should have a working calendar, and my org-mode files got too cluttered, and evolution/thunderbird/korganizer all feel too heavyweight.
I think apple did a good job with the versions of address book & icalendar on the versions of OS X I'd used 10.1-10.6, and would like some functionality like that on ubuntu. Since I've spent the last year or two trying to rebuild one of my work applications around the semantic web stack, gnome tracker's use of sparql appeals to me. And since some chunk of the N9's PIM stack was integrated with tracker, it seemed like a fun place to poke.