When using the iPad VGA adapter, you don’t get mirroring of the iPad display. Of the bundled apps, only the Video, YouTube, and Photo can display anything on the external display. Even then, you only get anything when you play a video or start a slideshow. Well, except that videos I downloaded from iTunes or “digital copies” of movies won’t work. The iPad claims the external display is not authorized to show them.
This is one area that is not a magical experience.
Keynote will also use the external display, but only when “playing” the presentation.
The last presentation I did involved slides followed by a demo. The demo used a web browser and an SSH client. On my iPad, I have Keynote, Safari, and iSSH. I have everything I would’ve needed...except that Keynote is the only one of those apps that can use the external display.