I recently upgraded WordPress to 3.0.1 and immediately got this:
Error! No TTF font found!
I got this all over my site and even the WP dashboard.
This was the result of the Captcha! ver 2.6 plug-in.
I turned this off and the Error went away. I'm a noob so I don't know why this happened but if people are running into this error trying turning off Captcha! It can't hurt.