From their FAQ:

        * Google Chrome: version 4.249.4+
        * Mozilla Firefox: version: 3.5+
        * Apple Safari: version 3.1+
        * Opera: version 10.5+
        * Microsoft Internet Explorer: version 6+

        The Google Font API is not currently 
        supported on iPhone, iPad, iPod, or Android

It's based on @font-face, which has been around for a while, but stalled over worries about font licensing, and disagreements over which font formats to use. It's slowly been getting support and is now in most major browsers, though I believe IE and non-IE browsers (as often) still don't support quite the same set of font formats, which is one thing the Google Font API wraps.

Here's the w3c's page on @font-face itself: http://www.w3.org/TR/css3-fonts/#font-resources