Call them back today!!
FearlessFreep:
Please call them every day until you finally talk with someone who can give you a definitive answer. The most likely reason is an administrative snafu or a lost letter. Unfortunately, I have seen these kinds of things happen to some of my clients at nearly every airline. The HR departments are pushing LOTS of paper, and things do slip through the cracks sometimes.
For all of you guys:
Reality check: The folks pushing paper at airline HR departments, People Departments, or whatever they call themselves, are usually not what you would call "seasoned professionals." They are typically young folks in their first/entry-level job at the airline. They are probably more concerned about which bar they are going to that night than whether your application file is kept immaculate. The things which you consider more important than life itself are not necessarily that important to them. Bottom line: you are nothing but a faceless name and SSN.
If something doesn't seem right during your application process or the pre- or post-interview processes, then CALL THEM! Be polite and tactful, but do not let them just blow you off. If you remember someone's name from your interview, then ask for them by name.
It is never normal to NOT hear from a company you interviewed with. Follow up until you know where you stand.