I remember something like this happened at AV years ago. During an incident dubbed as AV's Black Monday, a bad index push caused lots of sites to be dropped from the index. The incident was one in a long sequence that got other search engines, such as Google, attention. I'll note that on the various blogs, forums, etc. where this is being discussed, people have indicated that there were no such problems at Y!
The quality of search results for common queries is so close that an engine cannot afford to give its users any reason to search anywhere else. But this snafu has given some users reason to go to Y! (and perhaps some of the others), and notice that their results might be just as good after all. It will be interesting to see what happens next – will Y! and some of the other engines pick up some traffic? Or are people so used to Google that it will continue to dominate?
On a related note, there have been other recently reported instances of poor Google results. For example, search for [stock market] (no quotes), and check for that Neopets result.