Yes they ARE covered under warranty. My 1.9 Mjet nuts were replaced at 26 months last November, along with the rusting locking wheel nuts. I 'phoned Fiat Customer Services UK - they did not even ask for an inspection or photographs - they just booked me into my local dealer. When you get the new ones, remove them one at a time & coat them with "Waxoyl" otherwise they will also rust!! All Fiats/Alfa Romeos/ Maserati's & even Ferrari's have this problem. The Italians just paint bare metal but the Germans anodise there nuts!
That's good to know...
Not sure whether wheel bolts are anodised?