In the product catalog that comes with Wrath of the Lich King, Blizzard misspells Arthas as "Arthus" on page six. It's pictured above for those of you not digging through your stuff to grab the catalog and see for yourself.
I'm not going to criticize the folks at Blizzard for making a spelling mistake. God only knows that I've made a number of them myself. But usually I do them without an editor or reviewer. But that's just me. I don't know enough about the situation at Blizzard to really comment otherwise.
None the less there it is, and you're free to make whatever assumptions you may make about it.
Thanks to Knuxx and Taeous for the picture and tip.