This excerpt taken from the CPHD DEF 14A filed Mar 20, 2009.
Any employee, including an executive officer, who is employed by Cepheid prior to the beginning of an offering period, is customarily employed for more than twenty hours per week and more than five months per calendar year by us, and who does not provide services to us as an independent contractor and has not been reclassified as a common law employee for any reason other than for federal income and employment tax purposes as of an offering date, is eligible to participate in the ESPP. An offering date is the first business day of a given offering period.
As of the record date, approximately 393 employees were eligible to participate in the ESPP. The ESPP is only available to our U.S.-based employees.