From http://dev.live.com/virtualearth/sdk/:
VEMap.SetMapStyle(mapStyle);
Parameters
Parameter Description
mapStyle The style of map you want to display. You can specify the VEMapStyle enumeration or a string value. Valid string options are a, r, h, or o.
VEMapStyle.Road
r
Sets the map style to a traditional road map
VEMapStyle.Aerial
a
Sets the map style to aerial (satellite and aerial imagery)
VEMapStyle.Hybrid
h
Sets the map style to a combination of aerial images with labels overlaid (also known as hybrid)
VEMapStyle.Birdseye
o
Sets the map style to use bird's eye imagery
OpenLayers
