I just received an email notifying me that I am an alternate. Here is a section of it:
"We received more than 3,200 completed applications which were subjected to a rigorous external review by distinguished scientists. Selections were based on demonstration of outstanding academic achievement, graduate research and career objectives, and letters of recommendation. From the review process, 450 applicants were selected as Finalists; 150 of the most highly reviewed Finalists have been extended a fellowship offer. A prioritized list of Alternates has been established and you are among this group. "
I've got till May 5th as well.