Under those circumstances, no, they could not take money out of the 401k accounts, and they could only stop making the matching contributions if the plan document clearly states that they are discretionary. This is would be so unbelievably illegal that I can't believe the employer is seriously considering it - that sounds like rumor to me. And if they really are considering it, swalsh is correct - the plan administrator would stop it before it happened. 401k plans are audited regularly and you would not believe the size of the fine the employer would be hit with, or the number of people who would go to jail, if they were to go ahead with this.
So worry about layoffs, but don't worry about the 401k being decimated. Isn't going to happen unless the employer has a death wish.