Does that work in IE6?
No, IE6 does not support attribute selectors at all, cf. CSS Compatibility and Internet Explorer.
You might find How to workaround: IE6 does not support CSS “attribute” selectors worth the read.
EDIT
If you are to ignore IE6, you could do (CSS2.1):
input[type=submit][disabled=disabled],
button[disabled=disabled] {
...
}
CSS3 (IE9+):
input[type=submit]:disabled,
button:disabled {
...
}
You can substitute [disabled=disabled]
(attribute value) with [disabled]
(attribute presence).