this seems to work... but I am not sure how I can get rid of the <g:SimplePanel addStyleNames="{style.gwt-DialogBox}">
<ui:UiBinder xmlns:ui="urn:ui:com.google.gwt.uibinder"
xmlns:g="urn:import:com.google.gwt.user.client.ui">
<ui:style>
@external .gwt-DecoratorPanel;
.gwt-DecoratorPanel {}
@external .gwt-DialogBox;
.gwt-DialogBox {}
@external .Caption;
.gwt-DialogBox .Caption {
border-top: 0px;
}
</ui:style>
<g:DecoratorPanel>
<g:VerticalPanel>
<g:SimplePanel addStyleNames="{style.gwt-DialogBox}">
<g:Label addStyleNames="{style.Caption}" text="Panel Title" />
</g:SimplePanel>
<g:Label text="content goes here"></g:Label>
</g:VerticalPanel>
</g:DecoratorPanel>
</ui:UiBinder>