Is it possible to use an iPad as the monitor for a Mac Mini? Input would be from the Mac Mini's keyboard and mouse.
You should be able to set up the VNC server on the Mac (it's under System Preferences > Sharing > Screen Sharing), and use a VNC client on the iPad.
This goes over the network, not through a cable. It's possible there's an app and cable solution, but I haven't heard of one.
I just found this post, which describes how to set up the Mini side of it.
Alternatively, there is Air Display.
You can set it to mirror the desktop and it will act the same as a primary monitor for the Mac Mini. I'm not sure if this will be faster or slower than VNC, though.