Ruby DTrace (on OSX Leopard)

Has anyone had any luck getting it to run?

Neither Instruments nor dtrace -l shows up any of the Joyent probes
that are supposed to be there. I guess I'm just doing something wrong,
but I can only find little blog articles with people cheering that OSX
contains DTrace, no examples of it actually in use.

Running the example script in /Developer/Examples/Ruby/DTrace yield me:

dtrace: failed to compile script ./print_calls.d: line 4: probe
description ruby*:::function-entry does not match any probes

Any ideas?
   -tim