GM Shelved It
There's a similar thread going on in another Forum here at SaabCentral and I posted in more detail there.
I had read on another board that GM shelved it and that the prototype is currently literally gathering dust under a staircase in Trollhattan.
I don't know details of why the engine wasn't adopted, but I did read that once the engine was in full production it was predicted to cost the same amount to build as a standard engine, so cost can't be a contributing factor.
I don't know if there were any technical problems that couldn't be overcome that we haven't heard about.
Or conspiracy theorists might figure GM couldn't fit an engine into its master plan that puts out the output of a V6 (225hp) while only sipping fuel and outputting minimal emissions.