I've created a NOOBs image which contains all the required data to include RetroPie on the NOOBs boot up menu. Took me a few hours using Ubuntu to create the Tarballs and its fully tested and working great on a Pi 2 Model B.
Its something I've noticed a lot of users are asking about but no one seems to really have a solution other than create the image manually.
I decided to use NOOBs V1.4 as a base with RetroPie V3.0 RC1 as it uses a new RetroArch config setup which means you only have to configure your pad once and it works with everything, no more manual RetroArch config and I included the latest Rasbian Distro too.
File size is 1.13Gb and its uploading to my Google Drive as we speak, i will update this page with DL links, Virus Total scan and MD5 when upload is finished.