While the module I posted above does seem to work fine in production, there's a wrinkle - simulating the cart completion via the relay response in authorize's context doesn't allow reporting of Ubercart data to Google Analytics via the standard submodule. I haven't thought of a reliable workaround for this so far.

However, I note that authorize has just announced a new integration method they're calling Direct Post (DPM) that may well supersede the SIM approach for Ubercart. The payment form is generated by the merchant site but posted directly to authorize, which then uses a provided return URL on the merchant site to post the results and generates a redirect to the merchant site on success. The result would appear fully-integrated but would still avoid PCI card-handling issues on the merchant system. Anyone already working on that?