In today's pop culture, what does it actually take to 'co-write' a song? (http://www.cnn.com/2003/SHOWBIZ/09/18/showbuzz/index.html#1)

musical talent? rhythms? chord progressions? melodical contouring? or the ability to say, "gee that's catchy"?

or "your label thinks it would raise your 'street cred' if the liner notes said you helped write it."