On JM in the AM, once, I heard a leading chazzan -- sorry I don't recall the name, but he was a leader in an organization for chazans -- make the argument that it is assur to adapt melodies from pop culture and use them when davening from the amod. My first reaction was, "I thought the melody many shuls use for Adon Olam was an old French marching song." My second thought was that in Modern Orthodox synagogues, I occasionally did heard the liturgy sung to an occassional pop tune, and that Lenny Solomon had written an entire album of Beatles melodies to words from the siddur. But then again, I've heard rabbis say that Schlock Rock, Variations, and the like, are more suited for baalei tshuva than Jews from a yeshivish background. Then I remembered the controversy when Lipa Schmeltzer adapted a pop song for his song "Uman, Hey," although I frankly never heard the original. Also, pop music sometimes creeps into our Jewish musical vocabulary unknowingly. My son-in-law tells the story of his Rebbe who was humming a Simon and Garfunkel tune and had no idea that it was a pop culture since he had thought it to be from Jewish liturgical music. Does anyone know the sources for these restrictions and to what degree, if any, there is consensus?
See Rabbi Student's excellent treatment, "Kedusha of Roses", here.
It's discussed; hundreds of years ago there were discussions about church music; Rabbi Moshe Feinstein says listening to gospel tunes without the words is distasteful but permissible; Rabbi Y.H. Henkin shlit'a concludes quite reasonably that it all depends on what will/won't distract the congregation. (His essay in Bnei Banim -- linked above -- is marvelous, and has a full treatment of the subject.)
A Sephardic friend of mine informs me that the Sephardic world has been more open to external music, with R' Ovadiah Yosef shlit'a punning -- al aravim betochaH talinu kinoroteinu -- our harps [of liturgical music] are supported by the [musical traditions of] Arabs.