No, it's not a doctype issue. Basically, if you want to use CSS3 at this stage you have to live with the error messages (unless you use a CSS3 validator, which you can choose as an option when you run the validation). Remember that validation isn't an end in itself, but a way to check that you haven't made a serious mistake. In this case, you just know that the error can be ignored.
That filter you have is (I believe) only for IE6 and 7, so you may want to add one for IE8: