If building an ICF basement and first floor cape cod style home, with SIP roofs, what would most people do for the 2nd floor gable end walls? Can that point be done in ICF, or should it be SIP instead? Thanks.

ICF gables with SIP roofs are done every day. Without the extra joint in the wall it's both structurally stronger and makes for one less transition seam to air-seal:

http://media.merchantcircle.com/33254646/IMG_2421_full.jpeg

http://www.icfinstall.com/blue80blog/?page_id=31

http://www.lockingtonhomes.com/galleries/nudura_gables.php