it will void your warranty but to restore it is quick and easy. just update your software to the latest version or just restore it and your back to original os. then you can claim your warranty. (besides, normally an iphone will last longer than a year at least if there is a problem... unless you have apple care for 3 year warranty)
iv gone from
iphone
iphone 3gs
iphone 4
iphone 4s
iphone 5
never had a single problem provided you DONT DROP it. hehe