When did Arabic enter into usage as a liturgical language among Christians? Today, many Catholic and Orthodox Churches in the Middle East use Arabic as the language of the Divine Liturgy, often mingled with Greek. When did this practice begin?

I assume that at the time of the Muslim invasions the only liturgical languages in use were Greek and Syriac, and I can't imagine that the Orthodox Christians at that time were too keen on immediately adopting the language of the conquerors. Nonetheless, this clearly happened eventually. How and when did this shift occur?
Different communities adopted Arabic at various times. The earliest community to start using Arabic were the Greek Orthodox of Palestine, who started translating the liturgy and theological books into Arabic in the 8th century. For a more general history of Arab Christianity, I'd consult The Church in the Shadow of the Mosque by Sidney Griffith. For evidence of possible pre-Islamic use of Arabic as a Christian liturgical language, browse through the series Byzantium and the Arabs by Irfan Shahid.