Employers of all sizes and types have struggled with the implementation of the Patient Protection and Affordable Care Act. But as the health care reform law has been rolled out, few have faced bigger questions than the staffing firms that supply a wide range of short- and long-term workers to U.S. companies.

One of the biggest puzzlers is whether staffing firms or their clients should be considered the "common law employer" of temporary workers. The common law employer is responsible for health benefits, and traditionally, the obligation lay with the staffing firms.

The language in final PPACA rules issued by the IRS, some analysts say, raises questions about whether the employer mandate even applies to staffing firms.