At long last, PLinq has finally found its way into Mono trunk as what can be considered a preview release. The code has seen a major rewrite compared to the initial implementation done during earlier Google Summer of Code which lacked flexibility and correctness.
Public API is complete except some operator still not implemented. In addition and as implied by the preview title, the current code hasn't been tested against all possible flavor of query so it might or might not work for you (i.e. deadlock). If that's the case, don't hesitate to fill a bug report with a query reproducing the problem. The more query I have, the more robust we can shape future release.
By the way, these PLinq performance tips still hold true.