There is a very nice Scribble application that allows you to draw on any modern phone / tablet / iPad or PC.
You can try it at www.wintellect.com.
Here is the same application rewritten to suit any size phone, with a button for “Clear”, and with changes because my daughter wanted a finer sized pen. Unfortunately it loses the nice chalkboard for simpler sizing.
It has been trialed on an iPad and it doesn’t work. The iPad doesn’t support “drag and drop” which means you can’t draw by moving your finger (or a stylus) around. I’m guessing it’ll get fixed in an update at some point. There are things other than Scribble which need you to be able to drag.
The latest version looks like:
You can get it here