I have spent the whole day battling with what seems to be a SharePoint bug – the “created by” and “modified by” search mappings in advanced search never actually index the relevant metadata, meaning that you cannot usefully search on either of these. Looking at the meta data properties returned on items from a fulltextsqlquery, createdby and lastmodifiedby information is always blank – meaning that these properties are not indexed. This is pretty surprising as these criteria are available on the advanced search on by default. Note that apparently these property mappings do work for legacy (SPS2003) content, see http://www.networkworld.com/community/node/19021.
From what I’ve read on other blogs, the current workaround is to manually map meta data properties, see http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1937319&SiteID=1. You can also use the author property as an alternative to the createdby property.