Some links to things that you might try with Rhino:

http://www.grasshopper3d.com/forum/topics/how-to-fill-a-shape-with

http://jarek-rhinoscripts.blogspot.com/2010/01/image-based-circle-packing.html

http://tobesch.wordpress.com/2010/02/15/circlepacking-within-curve-boundary/

- Michael