factory reset won't delete root, because it will only delete data, cache and i think sdcard partition. root is on system partition, so why should it dissapear?
OTA2 deletes /system/app/superuser.apk and /system/xbin. that's the reason why you lose root when updating. rootkeeper protects su file from /system/xbin by making a backup copy to /system named su-backup. superuser.apk can also be downloaded in the market. so you can restore root
present: Galaxy S3 i9300
past: iPhone 4S, Motorola RAZR, HTC One S, Samsung Galaxy S2, iPhone 4, Samsung Galaxy S, Nokia 5800, Nokia 6700 classic, Nokia 6500