?You should be able to calculate it yourself, quite easily.  Recall the derivation of a least squares fit curve using calculus.  Just add up the square of the difference between each data point and what it "should be" (the other line or set of data points that you mentioned?) to get the sum of the square of errors and correlate from there?... it can be done manually once you have the definitions of what you are looking for.  I doubt is is built in .. however, defining what you need and programming it into Excel shouldnt be that tough?
Good luck...

One thing that should be carefully considered is potentially improper application of least squares regression.  

The basic premise is equal weighting of all errors.  This may lead to large errors if the data is highly non-linear or if there is a large dynamic range in the data.  Linearization prior to fitting may prove to be more useful.

TTFN

try the CORREL function...

Thanks melone - that's exactly what I was looking for

It works in StarOffice anyway, that's all I have access to at the moment.