finally figured this out.
The ruby that ships with leopard is "frameworked" by Apple and linking with it creates plug-ins that will work at the OS level but crash sketchup.
I downloaded the regular version of ruby 1.8.6 and installed into usr/local/bin and then changed RUBY_HOME to point to the new one.
Now my compiled plugins work with Sketchup on Leopard!
Finally I can get back to the hard part of porting sketchyphysics to the mac...
Kevin