In Safari 3.1 the offset is misreported for position:fixed elements inside any positioned element. The offset of the container element plus the offset from the document is returned, rather than just the offset from the document. This problem still exists in the latest Webkit nightlies. The bug does not occur in Firefox 2 or 3, nor IE7 (not applicable in IE6, which support fixed positioning).

I've demonstrated this in a testcase here:

http://www.babywhale.net/offset/