Besides the well known hexagonal (wurtzite) phase the group III-nitrides can also exist in the metastable cubic (zincblende) modification. One of the distinguished physical properties of the cubic nitrides is the absence of electric fields due to spontaneous polarization or piezoelectric effects in heterostructures. Therefore the cubic nitrides are supposed to be well suited to tackle the still open question of radiative recombination in InGaN/GaN quantum wells. In this paper we review recent experiments which demonstrate the importance of highly localized quantum dot-like structures formed by phase separation in InGaN for the optical properties of InGaN/GaN heterostructures.