From ZDNet News: Laid-off techies invoke old law :

The Worker Adjustment and Retraining Notification Act, which requires companies to give employees 60 days notice before mass layoffs or a plant closing, was originally intended to help blue-collar workers deal with plant shutdowns. Now, laid-off tech workers are finding out that it can apply to them as well, and they're taking action.
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It requires companies with more than 100 workers to give employees at least 60 days notice of a plant closing or mass layoffs. A plant closing is defined as the shutdown of a single site in which 50 or more employees lose their jobs. Mass layoffs occur when a company lets go 500 workers over 30 days, or 33 percent of the work force, if that means at least 50 people.