Looks like avr2012 already confirmed what I was going to say, but I did speak with a professor at SAIS who said that you can take the econ waivers and should you fail, as long as you still are able to sign up for pre-term you will be able to fulfill your intermediate micro requirement for IDEV.
I was told the waiver is a 25 question multiple choice exam. Sounds as though if you've had a good amount of econ background you will be fine on the exam, just make sure to review beforehand. I would think if you were an economics major you should definitely try to take the waiver if you have a little time to review before. Worst that happens is you have to go to pre-term.
hope that helps!