A few hours ago the Nebula Awards site posted revised rules for the award, which will take effect this month (meaning 2008 stories will still be considered under the old rules). As stated on the SFWA livejournal page, "Some of the key changes include: the elimination of rolling eligibilityand a return to the calender year system, the elimination of the script category as a Nebula Award, and a new, streamlined process for getting to the Final Ballot."
As an active member of the SFWA, let me publicly state I like these changes, especially since the final ballot will consist of the six stories in each category with the most nominations. While this could open the award up to some gaming by SFWA members--i.e., you nominate stories by your friends, they nominate yours--I believe this system will allow less gaming than under the old rules. And getting rid of the rolling eligibility is a great thing. As Tobias Buckell says, "I think bringing the Nebulas to a calendar year system will make them something to pay attention to as a reading list for people looking to find the field’s interesting work." Amen to that.