I would question, though, whether this is really something we want to be requiring employers to provide. Insofar as paid maternity leave is something we think we need (and it should be!), funding it is a shared responsibility that government needs to undertake, not employers. If your welfare state rests on a foundation of coercing corporations into providing benefits, you're probably going to wind up with a lot of economic distortions and a pretty crappy welfare state to boot.
But a great model for "shared responsibility" already exists. Senators Chris Dodd and Ted Stevens have proposed setting up a family/medical/maternity leave insurance system into which employers, workers, and government all contribute. Here's how it would work: Employers and employees would each contribute .2 percent of the worker's salary to the fund, and the federal government would pay all the administrative costs. Workers would be guaranteed up to 8 weeks paid or partially paid family/medical leave, tiered according to their income. For example, an employee making under $20,000 would receive 100 percent of their salary while on leave, while those making $61,000 to $97,000 would be entitled to only 40 percent of their salary. Those making more than that would not be guaranteed paid leave.