The in-store checks seem to be (in practice, anyway), a matter of the sales clerk's belief that you are likely a student. If you appear to be in your teens or early twenties, then they may apply it without asking....
I've been in Apple stores plenty, and I have yet to see anyone
not ask for student ID when someone asks for an edu discount. Even when I was getting AppleCare for my MacBook a few weeks ago, the guy asked me for a student ID after flat out asking me if I was a student or teacher anywhere. I've always been asked every time I try to get one on some hardware be it an iPod or a laptop. So has everyone else I've seen while waiting in line to purchase something. Last time I was at the store, I even saw one cashier befuddled over an out-of-state student ID..
I would presume your acceptance letter would work...
I believe acceptance letters were acceptable as proof of being a higher ed student according to the apple policy on edu discounts...
since when can u get a edu discount on a ipod!?
i found that out the hard way while trying to buy my mom an iPod for mothers day

I thought Apple had discounts for all the iPods for edu, particularly cause i think the terms and conditions for online edu still mentions something like infinite discounts on iPods during the academic year... Their summer edu deals seem to involve iPods nowadays, and when I got my last iPod it was through an education thing
Long story short, falsifying your student status is fraud, no matter how you look at it. All it takes is a couple bad eggs to spoil it for everyone.
Actually, it doesn't matter. If you want an edu discount but don't have ID on you in-store, you can go back with the appropriate ID and such to the store within a couple of days to get the discount even after initial purchase...if you lie about the discount, Apple'll bill you for the difference later.
It's fraud, but if they discover it...the worst that'll happen is you having to pay it back. Not that I condone it, because I love education discounts and they've saved me a pretty penny while buying Apple hardware and if too many people lie...not a good thing...