When an element is a descendent of a containing block with visibility: hidden, it gets hidden too. This is not correct. The descendent should respect it's own visibility property, regardless if it's containing block is hidden. The bug is apparent when both elements are positioned and the container is the descendant's containing block (the descendant does not have position: fixed).
Created attachment 18124 [details] testcase showing problem
I can reproduce the bug on the version of Safari shipping with Leopard, but not when using a nightly build. I think it has been fixed by <http://trac.webkit.org/projects/webkit/changeset/27651>. Please use a nightly build from <http://nightly.webkit.org> to verify that the bug is fixed.