Here's a pie chart showing "Pages Using Custom Fonts":
http://httparchive.org/interesting.php#fontsIt shows that, as of Mar 15 2012, 8% of sites use custom fonts. This is based on HTTP requests. It's calculated by dividing the number of sites that have at least one font request divided by the total number of sites. A request is considered a font if:
response Content-Type contains "font"
OR
the url is like (MySQL syntax) "%.eot" or "%.eot?%" or "%.ttf" or "%.ttf?%" or "%.ttc" or "%.ttc%" or "%.woff" or "%.woff?%" or "%.otf" or "%.otf?%"
OR
url starts with "
http://use.typekit." and the Content-Type is "text/css" (they inline their fonts as data: URIs)
This logic is found in dbapi.inc in the function computeOther().
-Steve