2007.02.05 17:54 "Re: [Tiff] tag for tags", by Phil Harvey
In a TIFF image, IPTC information is stored in tag 0x83bb. In a JPEG image, it is stored inside the Photoshop IRB resource.
But your best bet is to use XMP and write the keywords to the Subject tag in the Dublin Core schema. The XMP information is stored in TIFF tag 0x2bc.
XMP is gaining support and IPTC is slowly becoming obsolete.