Not quite CUDA, but I've written a very simple OpenCL wrapper library 
for Ruby called Barracuda.

http://gnuu.org/2009/08/30/barracuda-an-opencl-library-for-ruby/

It currently only works on OSX 10.6 (Snow Leopard), but if you have the 
OpenCL libs installed on your machine and are willing to hack the 
Makefiles for a few minutes, you could probably get it to compile for 
other platforms.
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