This quotation describes the circumstances under which NSF will pay for you to take the GRE. Full context:
"Graduate Record Examination Registration
NSF will pay Subject Test registration fees for applicants who register for the November 8, 2008 administration under two conditions: (1) the NSF Fellowship application is the primary purpose, and (2) the GRE registration form for the November test is received at ETS no later than October 3, 2008.
The following condition is imposed on the reporting of the November 8, 2008 GRE Subject Test scores when NSF pays the test fee. Prior to May 1, 2009 these scores will be reported only for the NSF Graduate Research Fellowship Program, to the fellowship applicant, and to the applicant