Frequently, the search will not reflect the latest entries in the Find dialog. Sometimes the search term "sticks", meaning that the previous term is used after a new one is provided.
The "whole word" and case options don't seem to work at all. I activated "whole word", and the search continued to hit word fragments in the code.
This was in a 64-bit build created and used in Xcode 3.2.1 under OS 10.6.2.