The Pakistan-Iran border neatly bisects an ethnolinguistic zone named Baluchistan and basically represents what parts of Baluchistan the British did (Pakistan) and did not (Iran) conquer. The last polity to own the entirety of Baluchistan was Ahmed Shah Durrani's Afghanistan-based Empire in the late 18th century, which fragmented into smaller realms after his death, in that region mainly Kalat. The princely state of Kalat was subjugated by the British in 1839 and what was once Kalat joined Pakistan in 1948, but the remainder of Baluchistan remained in Persia/Iran. The Baluchis tend to be Sunni (and thus not particularly keen on Iran's Shia theocracy) and have their own language that is very distinct from both Urdu and Farsi and Baluchistan is very mountainous and difficult for Tehran and Islamabad to rule, and there have been various proposals for an independent Baluchistan taking in both Iranian and Pakistani Baluchistan over the years, but for obvious reasons neither government is particularly keen on this.

EDIT: a handy language map from Wikipedia. See how Baluchi (Yellow) neatly straddles the border? http://en.wikipedia.org/wiki/Balochi_language#mediaviewer/File:Moderniranianlanguagesmap.jpg