I don't know much about 401a plans, but if I recall correctly although employers can put a bunch of money in for their employees pretax (up to $45,000 in 2007), there are circumstances where any employee contributions if allowed are post tax, not pre-tax. It may be that the employer has the option to decide what kind of employee contributions to allow. I do know that there are at least some of these plans which only allow post tax employee contributions.
I think the 401a plans are very flexible from an employer standpoint so it would also be nice to know how your employer intends on calculating the amount of money it will contribute to the plan on your wife's behalf.
EDIT: Also, these can be either defined benefit or defined contribution plans, so be sure you know what you are getting.
No more lawyer stuff, no more political stuff, so no more CYA