The federal law says:
A WARN notice is required when a business with 100 or more full-time workers (not counting workers who have less than 6 months on the job and workers who work fewer than 20 hours per week) is laying off at least 50 people at a single site of employment.You can get around this by having the layoff numbers be under the threshold at any one time, moving people around, and spacing the layoffs in waves.
Also, IBM has a program where they are transitioning to contract workers. So you're told that you can keep your job for a while if you move your job to a contracting basis. Then when contractors are laid off, there is no reporting requirement.