The notion that Beowulf's nudity in the movie can be justified by the original poem seems to be floating around out there on the web. I seem to recall Neil Gaiman himself making that claim in an LA Times interview several months before the movie opened, so perhaps that's the origin of the idea. (I can't find a link for this article, though.)
I'm afraid, though, that that's a misreading (or at least reinvention). The relevant passage starts at line 661.
Ða he him of dyde, isern-byrnan,