Do any of those schools send out separate MD acceptances or give you an automatic MD acceptance with your MSTP acceptance? If not, and you don't have an MD acceptance anywhere you want to fall back on, you may be in a tough siutation. I know that here at UPenn, the MSTP acceptance does not imply an MD acceptance (or even MD consideration), and turning down your MSTP may shut you out completely. At the same time, you could come here, be a student for awhile, and leave the MD/PhD program for the MD program.
Strange huh? I guess for some students with an extra-strong research background and high numbers, it's easier to get into MD/PhD than MD. As such, MD programs don't want students using a backdoor to get in. Though I think the real reason is that at many schools the two committees (MD/PhD and MD) are separate, and the MD program would have to review you before you could come if you decided to enter as a MD-only. It would be a late in the game decision and your MD/PhD acceptance would have varying levels of influence on the MD committee. Again, this isn't true for some programs that imply a MD acceptance with your MD/PhD acceptance or review you separately and send you a decision seperately.
That leaves you in a strange place. You could knock yourself out of med school by telling all these schools what you really want is MD now. One option is calling schools and just asking about this option and see what they say because you're undecided. They're not going to withdraw your acceptances for asking questions. This will probably give you a better handle on your situation. Let us know what they say.
Good luck!