" I think we're seeing a slow fade into obscurity."
This says it all. Yahoo was just not able to keep up when Google started picking up momentum, which in my mind was when gmail launched (it was for me at least, that's when I switched from my yahoo mail to my gmail). I see Yahoo going down the same path as blackberry and Nokia... they'll either be absorbed by another company, or will shrink there business scope further... but I don't think they'll be able to keep the lights on for much time.
IMHO, the reason bing is finding success is because of Internet Explorer...it's set up as the default browser, and many folks (including me) like using it (althought many times I'll switch between Google and Bing depending on what I'm looking for, but 8 times of out 10 bing I get similar results). Yahoo right now doesn't have anything specifically driving business to them (aside from the annoying "set yahoo as your defaul search provider" checkbox when installing programs)