Why's my 'Go' button outside of the form?

It's because you've floated the form footer div and now it's not constrained by the form wrapper. If you disable the float property in the rule on line 605 of your style.css file, you'll see that the button is once again contained in the form wrapper.

screenshot: http://bit.ly/rpagBS

If you want to float the button to the right, you can do it like this. You'll need to also add a height to the footer element so the spacing is retained.

[css]
body .gform_wrapper .gform_footer input[type="submit"] {
    float: right;
    margin-right: 20px;
}

body .gform_wrapper .gform_footer {
	height:30px
}

screenshot: http://bit.ly/mR1pcX

Wonderful. Thanks Dave.