Heat radiates in all directions and dissipates through the aluminum body. While clamshell mode or using a keyboard covering slightly inhibits dissipation, it's not enough to cause problems or raise temps more than a few degrees, since the primary cooling is through the vents.
There has never been any air intake or exhaust vents through the keyboard on any Mac notebook. For all Mac notebooks except the MBP-Retina, both intake and exhaust has always been through the vents in the rear near the hinge. The new MBP-Retina has intake vents along the sides at the bottom and exhaust through the rear vents, near the hinge.
Learn about the fans in your Mac
This arrangement allows for venting with the lid open or closed (for operating in
clamshell mode). There is a solid panel under the keyboard, preventing any meaningful airflow.