2009.07.14 17:47 "Re: [Tiff] TIFFTAG_RICHTIFFIPTC definition", by Phil Harvey
I agree that LONG is the wrong format for this information, but you do run into compatibility issues if you use another format. I tried. I know first hand that the Nikon Capture software barfs when the IPTC is written with as UNDEFINED.
Photoshop set the standard, and all other software has come to expect IPTC in LONG format. I have seen it incorrectly byte swapped by an application (once), and for this reason I have put in an explicit test in my software to handle this case.