I read this chapter http://booksandjournals.brillonline.com/content/10.1163/ej.9789004173330.i-358.39 and I am not so fond of what he says. He states according to Ibn al-Qifti, from which most scholars quote the alleged apostacy, said that RaMbaM read the Quran and participated in the prescribed prayers during the period in which he acted ostensibly as a Muslim. Ibn Abi Usaybi'a reports taht RaMbaM learned the Quran by heart and engaged in the study of Islamic law. Al-Safadi claims that on a ship in which RaMbaM traveled from Morocco to ArasS Yisroel, he participated in the tarawih prayers for the month of Ramadan. However, the chapter goes on to say that these allegations appear to be fallacious, and contemporary scholarly opinion is divided on the question of RaMbaM's forced apostasy while in Morocco. Ibn al-Qifti recounts how an Andalusian Muslim met RaMbaM in Egypt and saw that he reverted to Judaism. The case was brought before RaMbaM's patron, the qadi al-Fadil al-Baysani, who ruled the conversion to Islam under coercion was invalid and acquitted RaMbaM. Skeptics cite this as proof of the spuriousness of Ibn al-Qifti's account, since one who apostatizes from Islam is liable to death penalty. The author then brings that an Andalusian writer Ibn al-Attar states that if a dhimmi was coerced to accept Islam and later reverted to his former religion, he was not to be penalized, and this rule was repeated by RaMbaM's Andalusian contemporary, al-Yaziri. In Egypt, Christians and presumably Jews, who had been forced to convert to Islam under al-Hakim, were subsequently allowed by him and his son to revert to their former religion. In the Cairo genizah, there is evidence that Yamanite Jews who accepted Islam rather than execution in 1199 were permitted to openly espouse Judaism in 1202. The author then goes on to state the Epistle of Consolation by RaMbaM's father and the Epistle to Yaman by RaMbam himself which both talk about forced conversion of Jews to Islam and that these letters both show that both RaMbaM and his father were against Islam.
I am not a Arabic specialist so I don't know what is really said in those books by the Arab writers which were quoted, however I am sure there is hard evidence against them as well. I listened to a lecture about this topic: http://www.youtube.com/watch?v=J-Wt2g7D9ks and I forgot what they say about this conversion, however I do remember that it is not really proven and is really unlikely.