Just saw that there were pictures posted which show frostmourne as a legendary item. But many people think that this is just a fake created by blizzard.
source: http://www.wowszene.de/comment.php?comment.news.1195
http://my.buffed.de/fileadmin/mybuff...ure-large-.jpg
http://www.wowwiki.com/images/3/35/B...rostmourne.jpg