From what I have heard (I have not used the ADC but I've thought about it), you pay $99 for a student membership which entitles you to one discounted purchase. That's per lifetime. So if you have a non-Mac buddy, it might make sense to have him do the purchase for you or something so you can get another one some other time.
Once you have the membership, you can purchase from the ADC store. The link is here
http://developer.apple.com/membership/usa.html where it says the "hardware purchase program store."
The prices essentially I think are 20% off. I think they are generally equivalent to if you just purchased the unit as a refurb also. You can only make one purchase so if you want a bunch of things, make sure to tack them on the same order because you aren't coming back.
Also, I believe it's a slightly drawn out process. They *will* verify you are a student so they'll contact you and have you fax them info, etc.
Edit: Fixed link