Welcome to the Wardrobe, Sheyla! As has already been mentioned, the religion of Calormen is quite different from most of the branches Islam, but there are some (very few though - I can only recall one, and even then very faintly) which are polytheistic rather than monotheistic. As is obvious though, this branch (or branches) that I can recall (from Studies in Religion class I did at the end of High School) is very much a minority within Islam.
But clearly, it'd be fairly unrealistic to suggest there's a hint at Lewis being afraid of Islam. That said, the "faith" or "religion" of Narnia is clearly a relation to Christianity (after all Aslan is in our world too with another name), and I guess if people are looking for religions in the real world which could be seen as being represented by the "religion" of Calormen, one that does kind of spring to mind is that of Islam. One also could probably take them to represent Hinduism or Buddhism.
Regardless of what the "religion" of Carlormen does or doesn't represent (in our own readings and interpretations), I feel it's important to remember that that's not why it was written. Lewis didn't write Calormen to represent Muslims any more than he wrote about them to represent Hindus or Buddhists...
Anyway, thanks! You just got me thinking!
Welcome (again) to the Wardrobe!