Many people actually have been questioning if Rule34 was using Shimmie or Danbooru's software.

The short answer is, no. Rule34 was created using 100% unique PHP code (other than BMP processing) by a person named Geltas. He pulled his hair out for me, Lozer, to get this script working and serving as many users at one time that was possible within our current limitations. The only code that remains remotely the same are notes javascript (very little) and the HTML displaying the page. Everything back end, and the remaining javascript files were recreated to fit the site better. There is no current public beta of this image board software available yet, but in the future we may release it.

With that said, there is still a lot left to be done, and it all really depends if Geltas feels up to the task and isn't already bogged down by university work.