The network prob have their own terms and may just cover you for the term that you are with them for but if they see you have changed the firmware an been messing abt then they wont do it.
But nokia as a manufacterer will sort it for you so dont worry, if your provider doesnt fix it then give em 2 fingers!
Either route you go dwn ur gona have to wait 3-5 days or so before you get your handset back. So dont think that walking into a vodafone or any other shop is gona make it quicker. They just gona send it off to their repair centre somewhere.