If you have the full version

If you have the full version of Pi Cubed (not Pi Cubed Lite), you can go to the Settings application on your iPhone, scroll down to the Pi Cubed group of settings, select that, then select "Copy equations as" and choose "Image". This will switch the default copy-and-paste behavior from copying the equation as a text representation to copying an image of the equation. You should then be able to copy the equation and paste it as an image in any application that supports pasting images, like Mail. I can't say if Documents To Go supports pasting images, but you can try.

To my knowledge, my application is the only one that does this kind of equation typesetting on the iPhone, although it's been a while since I last checked. There is no custom keyboard for doing this, because you need to have a means of typesetting equations within the applications themselves. I think the closest you're going to get is the image or PDF export from this application.

Sunset Lake Software - Comments for "writing calculus exam... "

If you have the full version