Code W is explicitly for contributions to an HSA. From the instructions for Form W-2 (one of TaxGuyBill's references):
Code W—Employer contributions to a health savings account (HSA). Show any employer contributions (including amounts the employee elected to contribute using a section 125 (cafeteria) plan) to an HSA. See Health savings account (HSA).
The confusion may stem from the mention of a cafeteria plan in this instruction, but it's specifically referring to contributions to an *HSA* under a cafeteria plan. It certainly does not refer to amounts the employer or the employee paid for insurance.
