When I first installed SCCM 2012 beta into my site with SCCM 2007 running I had problems with the MP not installing which had me scratching my head for a while and what it turned out to be was rather simple. The SCCM 2007 server had tried to install the client on the new SCCM 2012 server and I had to download the SMS 2003 Toolkit and run the CCMClean.exe /all on the server which cleaned up the client of the SCCM 2007 server and then my MP took off and installed like it should have. I did turn off the discovery portion of the SCCM 2007 server so that would not happen again. But it was a simple enough fixed.
Til later just Roger