I read up a bit on this and it seems there is no yahoo WMS server, they are actually displaying the images using a javascript (or flash) API. So it does not appear there is going to be a WMS server/string you can just drop into any WMS application, like TopoFusion.
I am curious how they are doing and if there might be a way to do the same in TF. Will plan to take a deeper look into it.
Thanks for the note.