I think they are one and the same. In my opinion, there are crazy, mis-guided muslims just like there are christians. Like the christians that march with signs that say God hates fags, muslim terrorists are misguided in what their God wants also. They are still muslims, but they have lost their way.
A Muslim is one who submits. Submits to Allah, Follows the Sunnah of His Messenger, and struggles in this life, for the sake of Allah. Islamist is a term that was created to distinguish when one is talking about the "so-called Muslims" who are going around doing the craziest stuff in the world, and claiming to be doing it in the name of Islam. Which is in and of itself a contradiction.
So no, the two terms are clearly set apart. IF they meant the same thing, one would simply say Muslim. There would be no need to distinguish that one was referring to the radical-minded, irrational person behind the horrors that we see in some areas of the world.
An Islamist in my opinion, is one in the same as a Muslim. I will usually use the word extremist to accompany the word Islamist when I am referring to those who take Islam to extreme measures. But that is my interpretation of the word Islamist, and may not be for others.