I suspect in the case sensitive engines (which those would be I don't really know), all caps would mean better relevancy if the searches used caps lock when making the search, but worse if they used all lower case(what most people do). I can't see it helping for any other reason though.
Another thing to note is that using all capital letters is hard on a user in a psychological sense. Most people read words by reading along the top of the letters. The tops are very distinct for lower case and a single upper case letter here and there doesn't throw anybody off. If you try to read along the tops of UPPER CASE LETTERS, you will notice that they are all very similar, especially in a typed form. This slows down readers trying to read the words and is generally more difficult to read.