Google Operating System reports that Google Maps is testing out walking directions in some locations. I was personally not able to replicate the walking directions, but Ionut has a screen capture as proof.
We all know Ask.com had walking directions, but when Ask.com killed off their maps product, they also killed off the walking directions. Neither Yahoo or Microsoft Live Search currently offer walking directions.
There is a Google Groups thread discussing a possible walking directions feature, but that was all pure speculation.
I suspect there is a limited set of users included in the beta walking directions. If you are able to see walking directions for a search between 200 Madison and 200 Lexington, then you are included in the beta group.