it may effect warranty of some particular parts, as said eg wheel bearings, wishbones etc at they may argue thre would be an increase on the effort of these parts.
Perhaps even steering parts.
However for most things it should not effect it. They cant argue for example if the starter motor goes its due to alloys...
Most people experience is that warranties are not effected by mods unless the part that has failed is directly effected by the mod. and even then many dealers are helpful in anycase.
It would be unusual for the car to breakdown so that you cant drive it for parts potentially effected by alloys...
id not worry to much, i mean im sure they would sell you alloys as an option.
in short:
technically maybe,
in reality its most likely not going to be an issue.