CSS 3 will eventually include standards for code that tells browsers to temporarily download normal .ttf and .otf fonts needed to display text when they are not present on the machine, and the latest Safari, Chrome, and Firefox browsers already support this feature. Some additional info can be found here.
I've seen a report that Mobile Safari (e.g. iPad, iPhone) does not support embedded fonts, unless they are converted to SVG format.
I have put up a test page that should make your browser download the Gardiner font (assuming you don't have it already) to display some of the Egyptian heiroglyphs which start at U+13000 in Unicode 5.2. It may take a bit of time for the characters to show up.
This technique has interesting potential for enabling the display of uncommon language scripts without requiring the viewer to deliberately download and install special fonts.