Do the hit counters at ComScore have a different take on this situation? The Seattle Post-Intelligencer quotes a ComScore study that shows Microsoft's search share holding steady. However, that refers to October versus September search share. If you eyeball the one-year graph in that story, you'll see essentially the same decline that Neilsen found.
Beneath these two contradictory headlines, there is actually some consistent data. Microsoft has lost about 20% of its market share in the past year, but in recent months has seemed to stop the bleeding. Yahoo has even managed a slight uptick in searches over the past two months. It makes you wonder whether the now-unlikely merger of these two would make any difference when both are so far behind Google.