Find only works on "post-rendered" HTML text, and cannot find text in editor fields. For instance, it does not work here in this editor. This is a serious limitation when working with any on-line application, from GMail to the Wikipedia. Note that this has not been supported in previous versions of Safari either.
I believe that this bug is a duplicate of bug 7023.
I can confirm this is a huge issue for the campus I work at. We can't use Safari to effectively edit sites like our own in house wiki (wiki.umn.edu) without effective find.
*** This bug has been marked as a duplicate of 7023 ***