Windows 7 is slated for retail on the October 22nd this year. But… the full version of this latest Operating System from Microsoft made it to the Microsoft Developer Network Academic Alliance (MSDNAA) first!
MSDNAA is a network program for academic institutions to obtain Microsoft products easily and more importantly cheaply. Basically the certain Microsoft softwares are free for download on the network.
I am not sure about other schools, but if you are a student of NTU, you can register and get Windows 7 Professional! (Registration might take some time though.)
(Link) – Register for NTU MSDNAA
Updated (10/09/09): Download the x86 version (32-bit), it is the common version nearly everybody use. AND I am told that only NBS and SCE students can subscribe to it. – Thanks to Alan and Kenny for the reminder and correction.