nope Bizarre Creations, the franchise developer, was acquired by Activision during the development of PGR4 back in 2007. The rights to the franchise, however, remained with Microsoft. Bizarre went on to create Blur for Activision along with the James Bond 007: Blood Stone, which will be released tomorrow.
Forza developer Turn 10 Studios seems like an obvious choice for Microsoft, but Develop suggests that the project could end up with an out-of-house studio. Turn 10 is working on a Forza project that will utilize Microsoft's Kinect sensor in some capacity, which was shown at E3.