What about the Federal Employee Discount program? is that better than Edu prices?
Also, Amazon.com has a rebate going until Friday that'll get you $150 back on a 20" iMac. When you consider that it's also tax-free, it should be a good deal cheaper than Apple. Another little money-saving tip is to use A9.com for a few days (i think it took me 4 days) so you get Amazon's π/2% discount. it's only 1.57%, but for the iMac, it ends up being almost $30, which, conveniently enough, is just about how much shipping costs.
EDIT: it seems that Amazon now sells the iMac through J&R rather than Amazon directly, so the rebate doesn't apply.