Here's a collection of resources I found while surfing...
http://students.ed.uiuc.edu/strysick/arm.htm
http://www.pbs.org/teachers/mathline/lessonplans/pdf/atmp/ReachingNewHeights.pdf excellent!!!
http://revitalise.ncsa.uiuc.edu/workshops/c2_sess3/resources/ArmSpan_Guide.doc good for lead-in
http://www.learner.org/courses/learningmath/data/session7/part_h/homework.html good, but too advanced
http://www.his.com/~melodyl/acttwo.htm
http://www.shodor.org/interactivate/discussions/GraphingData/ good discussion on bivariate data
http://www.gifted.uconn.edu/siegle/Publications/GCTAnIntroductionToUsingSpreadsheets.pdf excellent for extending to coefficient of correlation and using Excel
http://www.iit.edu/~smile/ma9412.html lesson plan skeleton