IE11 : Unexpected behaviour: testing !document.all returns TRUE ?

I just installed the IE11 to test our web application with it.
After getting it working with IE10 we were dismayed to have to revisit this so quickly with IE11.

I was surprised to find that when testing in an IF condition,

!document.all returns TRUE (unexpected in IE11 - in IE10 it returns FALSE)
!document returns FALSE (expected)

We have a cross-browser checks such as

if (!document.all)
{
// do non IE stuff
}
else
{
// do IE stuff
}

We can work around this easily but it does not seem consistent and may cause much disruption for many applications/websites!

Is this a bug or a conscious design decision?

regards,
Anthony
Code Inline Ltd

So long...and thanks for all the fish!