QUOTE (410b @ Nov 4 2009, 01:11 PM)

I am assuming that participants mean all employees who have entered the plan, regardless of whether or not they have chosen to make contributions to it.
I believe that "participant" means as you say. (There may be other opinions.)
QUOTE (410b @ Nov 4 2009, 01:11 PM)

I am assuming the period involved is a plan year and that layoffs in two separate plan years do not have to be added together.
Not necessarily. Two separate layoffs could be part of the same event. For example, consider a CY plan year; layoff 1 occurs on 12/1/08 and layoff 2 occurs on 2/1/09. The two are probably part of the same event.
Other discussion about involuntary terminations "for cause" give credence to assuming these are "employee-initiated". It may help to discuss with qualified ERISA counsel. Since a partial termination always involves a subjective analysis of facts and circumstances, likely it will be important to have good documentation, of facts as well as conclusions.