I got my B.A. from a small regional school in the Southeast. Wasn't sure what I wanted to do (grad school or public sector), so I went into an MPA program. I was very clear during with my advisor that I wanted to potentially pursue a Ph.D. We had elective credits that you could use for specialized training (city management, human resource management, etc.), but they allowed me to use mine to take graduate course work in Political Science. However, the school I attended did not have an M.A. or Ph.D. in PoliSci, so I had to work out classes with undergrad faculty. They set me up with directed studies that were meant to mirror the level of discussion and reading that one would encounter in a Ph.D. program.
I am currently a first year Ph.D. student with no regrets. I'm happy I did the MPA and I am thankful that I had professors willing to work with me to get me prepared, understanding that my undergrad institution was not a prep school for advanced degrees. I was able to attend a small conference, SPSA, and MPSA, and I have a co-authored publication coming out later this year.
Obviously, CMU will be different. But if you can get a good advisor and some professors willing to mentor you, you will be able to work toward both goals without shutting either door.
And as jcross9 stated, if you really like policy and you are still unsure about the Ph.D., work at a think-tank. Also, the market for Ph.D.'s is terrible, so if you do pursue a Ph.D., you'll have the MPP to fall back on (that's my backup plan at least) if you can't get your dream job.
Oh, and congrats on the acceptances. Good luck!