Designing Pick-and-Place-friendly CAD library parts
The IPC-7351 standard defines a coordinate system origin for surface mount parts, as well as a standard orientation. There is a summary of the orientations on the last page of this document: http://landpatterns.ipc.org/IPC-7351BNamingConvention.pdf [Update: archived link], and illustrations in the full standard.
For most surface mount parts:

The origin is at the centroid of the part's body
The standard orientation is with pin 1 at upper left, except:

two-leaded components, where it's to the left
components where pin 1 is in the middle of an edge, where it's at top center



The reason for the origin being at the centroid of the part body is that pick and place machines use a suction cup to pick up the part at that point. Orientation on the reel can vary, so you have to tell the machine which way they are rotated when you load the reel.