Thomthom beat me two it but I'd typed it so I post it anyway
A plane can be determined by an on-plane point and a normal vector, or an on-plane point and two in-plane vectors, or three on-plane points...
See this http://code.google.com/apis/sketchup/docs/ourdoc/geom.html for notes on plane - a plane is set/got by an array of a point3d and a vector3d
so plane=[point_on_face, face.normal], OR an array of four values.
The array of 4 numbers gives the coefficients of a plane equation as follows:
Ax + By + Cz + D = 0
It consists of four coefficients A, B, C and D, where D is the constant term
The plane's normal vector is [Ax,By,Cz]
If D!=0 you can divide everything by D to get
a + b + c = 1 etc...
... this is a good source http://www.astro.uvic.ca/~tatum/celmechs/celm4.pdf
You don't really need to do too much with the plane's raw data - there are several API methods to get the distance a point is from a plane, or project a point perpendicularly onto a plane, make a plane from three points, find the 'line' formed by the intersection of two planes, find the intersect point of a line and a plane [a line is NOT an edge line=edge.line or line=[edge.start.position, edge.start.position,vector_to(edge.end.position)] (i.e. a point and a vector - a 'line' is infinitely long, as is a plane in two directions) etc etc.
