[skimbrel]

It has, but I bet this has trained a lot of Windows users to just click OK because it's another thing that gets in the way of loading a page…

At least Chrome's interface for this makes it less obtrusive than a pop-up dialog.

[Encosia]

In IE9, the mixed content warning is not modal. It blocks insecure content by default, with a notification bar that allows the user to allow mixed content.

[pyre]

IE9 hasn't been out for 'years' though.

[yuhong]

Yea, the mixed content warning in IE has been there since at least IE6 (likely earlier), but for IE8 they flipped the Yes and No choices probably because of exactly this, and as mentioned in another comment, in IE9 it is no longer a modal dialog at all.