I am using the XeTex typesetting system on a Macintosh running Snow Leopard. I would like it to produce a PDF/X file. Is this possible? I am mainly concerned with ensuring the fonts are embedded.
Yes, you should be able to, using the PDFX package. But you should realize that there is much more to the PDF/X spec than just embedding fonts. The PDFX package can be had here: http://www.ctan.org/tex-archive/macros/latex/contrib/pdfx/
Otherwise, produce your PDF with embedded fonts, then try and print the PDF to another PDF using OS X's print dialog, and save as PDF/X-3.
As far as I (not an expert) understand, XeTeX (and ConText) uses dvips as its engine. The dvips file is then converted to PDF. However, this cannot produce a PDF/Xn file (with or without a special package). The reason is that dvips simply does not contain certain data that must be in the final PDF/Xn file.
On the other hand, pdflatex can directly write PDF without the intermediary of dvips. With the correct package, pdflatex is instructed to write the necessary PDF/Xn metadata, before the document text even begins. Thus, pdflatex is capable of PDF/Xn output. Be aware that available TeX packages are not equally good at doing that.
If you must use XeTeX, then as long as your fonts are embedded, it is possible to post-process the PDF using professional software (such as Adobe Acrobat Pro), to create a new PDF/Xn file. If you don't have the software, and if your purpose is academic, your college department or library might have a license to the software (for academic purposes). Ask. Then you would get all the benefits of TeX, plus the PDF/Xn conformance.