The trademark covers exactly the "π" symbol displayed in the [Wiktionary page on Pi](http://en.wiktionary.org/wiki/pi#English) followed by a period ".".

The guy did not create his own font. Zazzle just started pulling all the things with the π symbol on them, because as I'm sure everyone is aware, nobody has ever thought to put π on a T-shirt before, so obviously this guy's trademark on "π." takes precedence over all the copycats!