Ubuntu – Using iPad as an external monitor

There have been projects to get operating systems other then iOS installed on the iPhone, however, it's not as simple as putting a bootable image on a USB and connecting it to an iDevice. For more information regarding this see:

• http://linuxoniphone.blogspot.com/
• http://linuxoniphone.blogspot.com/2010/04/ive-been-working-on-this-quietly-in.html
• http://code.google.com/p/iphone-linux/
• http://twitter.com/#!/planetbeing

There seems to have been a lull in development lately. The short answer to your question is that currently it's not possible to boot a Linux image from an iPad/iPhone.

Currently the only emulator for testing iOS applications is build for MacOS.

There is a work in process to bring an emulation software to linux which you might want to take a look at...but judging by the development status it'll be a while till it's usable.
http://www.iemu.org/index.php/Main_Page

EDIT: This is outdated information. The project was discontinued some time in 2013.

Your options at this time would be to try and install a hackintosh instance in virtualbox or use something like Appetize.io, if you have a bundled app that you want to try out.

For real productive development work I'm afraid you won't get around buying Apple hardware.