As Greg pointed out, this is not a straight-up faithful retelling of Norse mythology; it's a movie based on a comic book that has long played fast and loose with that mythology...

Doesn't matter what the Nordic traditions are, just what Stan & Jack and those who followed brought forth. That's the source material.
I don't really know Thor's corner of the Marvel Universe so I can't speak to whether this casting is a problem that will be pointed at later as a reason why the film sucked. I don't even know the actor's work.
It's not like Will Smith's son is the next Peter Parker although I've seen rumors...
