I have a Dell MFP1600n printer, and although the printer claims to support IPP and PostScript, every time I try to send a print job with graphics in it, the print job fails with the error message "Print file was not accepted (client-error-bad-request)!"
After looking into it for a while, it turns out that although the Dell printer does support IPP, you can't send generic PostScript over the IPP port: it will work for simple text printing, but as soon as you try to send something with graphics, it will fail out.
Fortunately the printer does seem to support raw postscript over port 9100 -- so here's something that works:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060225174600412