Measure of linearity between two variables?

Could you be more specific about what you want?

For example, do you want a statistical test with a significance 
probability or a measure like R^2?  Also, are your numbers bounded, 
e.g., between 0 and 1?  What kind of error structure is inherent in the 
application?  Should we think about transforming the response variable 
or using "glm" or nonlinear regression?

If I were concerned about saturation at either end, perhaps the 
simpliest thing might be to add a cubic to what you considered:

	x ~ y + I(y^2) + I(y^3)

 From this we could get significance probabilities for the squared and 
cubic terms combined.  Also, by converting the sum of squares column to 
percent, we get something like R^2.

Hope this helps.
Spencer Graves