IllegalPaladin on 29 October 2008
You could also register on the site and see if they still have any.
I followed a link (from Kotaku I think) this morning and got a code. I'm currently downloading the beta while I'm at classes.
*edit*- yea, it was Kotaku.
http://kotaku.com/5070206/call-of-duty-world-at-war-beta-is-go