I bought my Macbook last week at the local Apple store and asked about the student discount and how it works; the customer service rep explained to me that "you are representing that you're a current student at a university or college." However, I explained to her that I graduated last May (2006) and was going to be going back to grad school soon, and asked if that counted. She apologized and said no, and went back to the back to grab my computer while I shopped some more. I didn't want to be dishonest in any way and figured that paying full price wasn't so bad when you're getting an awesome machine, so I agreed and said that was fine, that I understood.
When she came back out she said she spoke to her manager and she'd sell it to me at the discounted price since I had been a student within the past year or so, and would be again very soon. I don't know that there's an exact science to it, and I think that they really do want to give us poor student types a break. I'm still paying off loans and simultaneously trying to save for more applications, tuition, etc. I thought it was really very decent of them.