I spotted a thread named Don't Get it? Case sensitive at Yahoo? at Search Engine Watch Forums which describes a case of conducting a search in upper case versus lower case brings back different results. The example given in the thread is a search on "Squaw Creek Condos" versus a search on "squaw creek condos" in Yahoo! Search.
Although I personally don't see any different results at this point in time, Danny Sullivan in the thread reports "First gets one match; second gets two!". Of course that sprung my interest, so I emailed Tim Mayer over at Yahoo! Search for some insight. He got to the bottom of it, telling me;
I looked into this and the only reason for this abnormality is that the document was not being returned in one cluster and it was being returned in another. Nothing to do with case sensitivity.
We often find slightly different results at Google when searching on the same term (no matter upper or lower case) due to data center fluctuations. Good to know we have a similar occurrence at Yahoo! Search.