As we understand it, for those employees who elect to purchase coverage through the exchange and receive a tax credit even though they have been offered coverage by their employer that meets the following basic requirements; 

• Coverage must be affordable (the employee pays less than 9.5% of their w-2 wages for employee only coverage) 
• Coverage meets minimum value (60% actuarial value of allowed costs)

the employer will not be penalized.

To be subject to the penalty the employer must employ 50 or more full-time equivalent employees (fulltime 30 hours plus part-time hours divided by 120) over any 6 consecutive month period in 2013. To avoid the penalty an employer with "large employer status" must offer the opportunity to enroll in health coverage to at least 95% of its fulltime employees and their dependents. Employers with fewer than 50 full-time equivalent employees are not subject to the penalty. 

For those employees who enroll in the Exchange and indicate they were not offered coverage or it didn't meet the requirements for affordability or minimum value, the government will contact the eligible employer to determine if the penalty applies. Employers are only required to offer coverage that is both affordable and provides minimum value. If an employee declines coverage and decides to go to the Exchange for coverage, and that employee receives a subsidy, the employer will have the opportunity to provide information that qualifying coverage was offered to the employee and that coverage was declined.

The DOL has provided a question and answer that directly answers this: (http://www.irs.gov/uac/Newsroom/Questions-and-Answers-on-Employer-Shared-Responsibility-Provisions-Under-the-Affordable-Care-Act):

The IRS will contact employers to inform them of their potential liability and provide them an opportunity to respond before any liability is assessed or notice and demand for payment is made. The contact for a given calendar year will not occur until after employees' individual tax returns are due for that year claiming premium tax credits and after the due date for employers that meet the 50 full-time equivalent employee thresholds to file the information returns identifying their full-time employees and describing the coverage that was offered "if any."

Employees who mistakenly receive a tax credit for health insurance premiums after their employer offered them affordable, minimum value coverage will be responsible to refund the subsidy if the IRS determines they were ineligible to receive the tax credit.  Unfortunately, since the determination will not occur until after the employee files his or her tax return the following year, the discovery could be over one year from the receipt of the initial tax credit.

The IRS is hiring thousands of new employees in an effort to manage the tax credit eligibility.  In order to avoid excessive reporting and employee confusion, it will be critical that employers communicate to employees their eligibility status for tax credits under the new federal Marketplace before the employee has the opportunity to enroll.