The Revision 2 board has holes for an additional header next to the main header P1 and also there are now mounting holes.

Wikipedia has a good graphical summary of the various board designs:

Raspberry Pi 1 model A+ revision 1.1:

Raspberry Pi 1 model B and B+ revision 1.2:

The Raspberry Pi 1 model B revision 2:

The Revision 2 board has holes for an additional header next to the main header P1 and also there are now mounting holes.