My Mac mini came with a DVI Male-to-VGA Female adapter, so if you can find a DVI Female-to-VGA Female adapter, then you just need a DVI M-to-HDMI cable (which are very easy to come by, and your Xbox VGA cable. Otherwise, you'll need to use entire cables in place of the adapter.
I would send e-mail inquiries to www.mycablemart.com and www.monoprice.com.
I wondering if anyone has ever heard of or know if there is such a thing that would enable me to hook up a Xbox 360 to a HDTV w/ only HDMI?
It appears microsoft only has a VGA cable for HD out of the 360. NO DVI or HDMI and my HDTV only has HDMI.