This is because submit
event know nothing about keys.
I'd advice to replace your submit input with button input. And there check will work fine.
Here is the sample:
http://jsfiddle.net/fFYKs/2/
<input type="button" value="Click here to submit" onclick='isPressed(event);'/>
And JS:
function isPressed(e){
if (e.shiftKey) {
window.myForm.submit();
}
}
Also for cross-platform details recommend to visit this link: http://goo.gl/wKdJMO