Because of the different technologies used by Mac's and PC's for complex scripts (AAT and OpenType respectively), it has long been necessary to use different fonts on the two platforms for languages written in Arabic script. Also AAT fonts are uncommon, so Arabic script on a Mac (unless you used particular apps like Mellel or OpenOffice) was essentially limited to the Geeza Pro font supplied with OS X and two fonts (Scheherazade and Lateef) provided by SIL.
Thanks to the Iranian Mac User Group (IRMUG), there is now available a new set of Arabic script fonts, XBZar, which contain both technologies and should thus display the same on both platforms and in all applications that support Unicode. XBZar, which has regular, bold, italic, and bold-italic typefaces, can be downloaded here.
Tiger (but not Panther) has some bugs that cause problems with this font. If you have an Intel Mac, you must be running 10.4.9 for it to work right. On a PPC Mac, 10.4.2 is sufficient.
For more info on typing Arabic script languages, see this page.