I can confirm this was just an utterly bone-headed error. I was just moving too fast, didn't pay attention. When you go to type in your WEP key, you need to look at the dropdown. By default it says "WEP Password". This ain't the same thing as a WEP key. So click that dropdown, there's an option for a straight WEP key. I felt so dumb when I realized that mistake.

The following article explains the issue: http://docs.info.apple.com/article.html?artnum=108058

Basically, if you are using WEP authentication, then for compatibility, use a password with 5 or 14 (ASCII) characters.