A short article on the Wall Street Journal site discusses saving money by using Free 411. It only mentions two sites: 1-800-Free-411 and Goog411.
However there are a wide range of free directory assistance alternatives:
There are some additional, smaller ones.
In our most recent mobile user survey (August, 2008) we found that a substantial majority of users had never tried one of these services. It's mysterious why they're not being marketed more heavily, given the opportunity to reach the majority of users who own non-smartphones (85-87% approx in the US), want information on the go and don't want to pay for traditional 411.
Microsoft in particular would be in a good position to gain usage for one or both of its two services and could potentially leverage those users into other mobile offerings.