Yup I'm in the same situation. Grabbed a one-way flight from MIA-NYC on a holiday weekend. The American flight was $384, but USAirways was showing the same flight for $124.
Only OTA I could find that let me book at that price was AirFastTickets (had never heard of them either, but linked from Hipmunk so I figured couldn't be too bad).
Ticket purchase seems like it went through... I got a USAirways confirmation code and can see the trip on the USAirways website (though shows that I paid in GBP, for whatever reason).
Now just need to figure out how to get the American Airlines record locator!
I figure I'll give it a day and then call USAirways to see if they know it.