This should work; make sure the message hasn't been filtered by some anti-spam software. If there's a boom of people signing up it may take a while, how long has it been? You sure you typed in the correct address...
This problem is more suited to .NET support, since they have a lot of experience in these matters. Fill in this form
http://register.passport.net/contactus.srf?lc=1033. In the field asking you for your .NET passport e-mail, put in the one you are trying to register with.