From page 11 of the W-2 instructions ...
Health savings account (HSA). An employer's
contribution (including an employee's contributions
through a cafeteria plan) to an employee's HSA is not
subject to federal income tax withholding or social
security, Medicare, or railroad retirement taxes (or FUTA
tax) if it is reasonable to believe at the time of the payment
that the contribution will be excludable from the
employee's income.
