Ian, Here is what I use for FlexViewer 2.0.
<basemaps>
<mapservice label="Streets" type="Bing" key="xxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxx" visible="true" alpha="1" style="road" culture="en-US"/>
<mapservice label="Aerial" type="Bing" key="Axxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxx" visible="false" alpha="1" style="aerial" culture="en-US"/>
<mapservice label="Mixed" type="Bing" key="xxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxx" visible="false" alpha="1" style="aerialwithlabels" culture="en-US"/>
</basemaps>
and for non viewer app
<esri:VETiledLayer id="Streets" name="Streets" key="xxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxx" visible="true" alpha="1" culture="en-US" mapStyle="road"/>
<esri:VETiledLayer id="Aerial" name="Aerial" key="xxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxx" visible="false" alpha="1" culture="en-US" mapStyle="aerial"/>
<esri:VETiledLayer id="Mixed" name="Mixed" key="xxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxx" visible="false" alpha="1" culture="en-US" mapStyle="aerialWithLabels"/>