The American College of Sports Medicine has released a new position stand called Appropriate Physical Activity Intervention Strategies for Weight Loss and Prevention of Weight Regain for Adults for 2009. Here is a summary.
Prevent weight gain

• 150-250 minutes/week of moderate-intensity physical activity is associated with prevention of weight gain
• More than 150 minutes/week of moderate-intensity physical activity is associated with modest weight loss.

Weight loss

• 150-250 minutes/week of moderate-intensity physical activity provides only modest weight loss.
• Greater amounts (eg, >250 minutes) provide clinically significant weight loss.
• Energy/diet restriction combined with physical activity will increase weight loss as compared to diet alone.

For weight maintenance after weight loss

• There is some evidence that >250 minutes/week of moderate-intensity physical activity will prevent weight re-gain.