Originally Posted by Peter Sorotokin
It is a part of SDK, however device manufacturers can override it (or even make it user-selectable) through user stylesheet.
My guess is that all vendors will be using the ADE SDK's default fonts. I attach a small ePub to test this. The screenshots are from the Windows Sony eBook Library viewer. The fonts look identical to these on my EZ Reader Basic and Pocket Pro (Hanlin V3 and V5), and also using Windows ADE.