QVFb is the "emulator" that Qtopia gets developed in. The power in this, is that it constrains your "screen" to specific display sizes, gives you control over what bit depths your display uses, and simulates buttons presses, through the use of a skin. This helps produce applications that are dynamically resizing, look good and actually work correctly in screens sizes as small as 176x220.
We don't need X! Of course, you don't need to use X to display Qtopia apps. If your device or desktop machine has a framebuffer device, you can display it there (if you are using X, try Ctl-Alt F2 and Ctl-Alt F7 to get back!). You can constrain the display size using QWS_SIZE. Of course, if you use this method, you will need to configure with -no-qvfb.
Developing apps in a virtual framebuffer is no more difficult to use that any X program. In fact, it's beneficial for developing properly behaving embedded applications for Phones and Pda's. Not to mention being able to get 'screen movies' easily.