Very important issue indeed !
Culture is based on a language and u can see that the speakers of a particular language share the same culture. In this way, we have as many distinct cultures in pakistan as many the languages we have. Major languages spoken in Pakistan are Punjabi, Sindhi, Pushtu and Baluchi.
But u will find one thing common in all these people, i.e. that they also speak and understand one common language, and that is Urdu. It means that a common culture is emerging of the mix of these four regional cultures and that is Pakistani Culture. Not only Urdu is the binding force of this pakistani culture, but many other common things (material/non-material) that are shared by all from Khyber to Karachi. Look the dress we wear, the food we consume, the entertainment we indulge in, the games we play, and many many more things we have in common.
Briefly, we can say pakistani culture is there and composed of several other regional cultures. This age of information techonolgy has accelerated the process of acculturtion and now we see the new generation of pakistan has a common PAKISTANI CULTURE with less influence of regional cultures on it. This is a very welcome sign from the point of view of National Integeration and soliderity of the nation.