What are the major issues facing society?
Some of the major issues are:

The environment (global and local) and concerns about contamination, degradation, remediation, loss of non-renewable resources, sustainable use, climate change, etc.
Population trends (demographics), impacts, and implications.
Globalization of the economy, the environment, information, science programs, etc.
Public health, including diseases, aging, drug use, and rising health care costs.
Education, especially science and technical education.

What is the role of science in addressing these issues?
Science has a major, if not critical, role in addressing all of these issues:

Many scientific disciplines contribute to understanding the environment, how the complex systems of the earth interact, how humans influence these complex systems, what the impacts of environmental changes are, how to make sustainable use of resources, methodologies for policy development, the provision of well-founded recommendations to policy makers, etc.
Science contributes to our understanding of population dynamics and trends, the politics of population, population control, etc.
Science can contribute to understanding the increasingly international nature of human endeavors, adequacy of existing institutions/infrastructure, etc.
Biomedical and social sciences are the largest contributors to understanding these issues.
The world is becoming increasingly complex, in large part due to scientific and technological developments. Thus scientific and technical understanding becomes more and more important for citizens to make informed decisions. Scientists and decision makers need to consider how to keep the general public from being left behind.

What are the major issues/challenges confronting science?
Issues that occur to me are:

Communication of the role and importance of science to societal issues
declining R&D budgets, including consideration of the role of the research university and the importance and role of graduate/technician training.
Dealing with ethical issues that arise as science enters areas of inquiry that threaten society's moral foundations, e.g., use of fetal tissue, reproductive technologies, genetic research, etc.
The wise and effective use of cyberspace.

What should the role of AAAS be?
AAAS is already addressing the following:

Expose scientists, engineers and the general public to the issues and problems facing society and how science contributes to understanding and resolving them.
Provide a forum where scientists, engineers and the public can have informed debate on issues where science and society intersect, e.g., natural resource management.
Communicate the importance of science to decision makers, especially since individual scientists do not seem to do a good job at this.
Provide visibility of science to the general public.
Develop educational/other programs to help the general public keep pace with ever more rapid scientific/technical advancements.

Introduction

AAAS at the Millenium Board Position Paper