Those who believe (in the Quran) and those who follow the Jewish (scriptures) and the Christians and the Sabaeans, any who believe in Allah, and the Last Day, and work righteousness, shall have their reward with their Lord, on them shall be no fear, nor shall they grieve. T.Q., Sura 2 of 114, The Cow, verse 62.
Nadia Lamlili has a very thoughtful post (in French) discussing the underlying similarities of Judaism, Christianity, and Islam and the potential for tolerance among them. Overall, she concludes that there is more room for tolerance than the adherents of the three religions are normally willing to admit.