Ok, just for the brave

After some search i decided to edit /usr/share/X11/xkb/symbols/nokia_vndr/rx-51 as specified in this thread and after a setxkbmap now I am able to tab checkboxes.

edit: @maacruz - we wrote last message almost together. My hw keyboard doesn't have a dedicated tab key (don't know if other layouts have it) and I had to recreate it as explained above. Thanks again for the app, I feel it will be really a good tool for my N900 and my job, too!