This doesn't answer your question exactly, but as late as 1377, in Ibn Khaldun's Muqaddimah, we see a distinction between actual ethnic Arabs and peoples who simply speak Arabic:

In that connection, "non-Arab" meant non-Arab by descent. Such non-Arabs had a long (history of) sedentary culture which, as we have established, causes cultivation of the crafts and habits, including the sciences. Being non-Arab in language is something quite different, and this is what is meant here.

Based on his writings, it seems that "Arab" in his contemporary society referred to the tribes coming out of the Arabian peninsula, whereas peoples living in Egypt, or Syria, were referred to as just that: Egyptians, and Syrians--as opposed today where it is quite common to refer to them as Arabs

Most of my knowledge on this subject comes from a great book written by Hugh Kennedy, [The Great Arab Conquests: How the Spread of Islam Changed the World We Live In] (http://www.amazon.com/The-Great-Arab-Conquests-Changed/dp/0306817403/ref=sr_1_1?ie=UTF8&qid=1414391328&sr=8-1&keywords=conquests+of+islam). I would highly recommend it for someone interested in the spread of Islam and Arabic language/culture.

To answer your question, I would say that there was very little ethnic expansion of Arabs during the early conquests of Islam, and what expansion did occur was much later and was restricted to areas in close proximity to the Arabic Peninsula (Baghdad, Damascus, Cairo, as examples). The predominant influence of the invading Arabic armies in North Africa, the Middle East, Persia, Northern India, was most definitely cultural. The invading Arab armies were content to appropriate their authority by conquering local lords and then introducing elements of Islamic law as well as conducting all official business in Arabic. The truly ethnic 'Arabic' peoples of the conquests that did not dwell in Arabia proper were almost universally warriors or administrators during the initial stages of the conquests; the expansion of the Arabic language and culture into places that we think of as mainstays today was a much more gradual process that involved 'Arabization' of the native populations with Islam and the written Arabic script.

Over time, this influence led to ethnically non-Arab peoples (like the Berbers, who had been around for a long time (1000+ years, at least) before the Arab conquests in North Africa) inheriting elements of the Arabic identity through the use of their written script and (eventually) the practice of Islam. White Europeans living in Hispania became painfully aware of this development when Berbers with Arabic names who practiced Islam (aka, the Moors) set up shop in Cordoba for the next 500 years. They were led by Tariq ibn Ziyad, an Arabized Berber who was likely born into slavery but went on to become the military leader of the Moorish invasion as well as the first governor of Al-Andalus.

I would say the exception to this rule of adoption of Arabic script and customs would be Persia, which already had an entrenched bureaucracy and a literate and educated populace relative to the other conquered peoples, along with a relatively strong regional identity for the times. The Arab conquerors had a significantly more relaxed approach to dealing with the Persians because of these reasons. It's not a coincidence that the Arabic influence in Persia, except for Islam itself, is largely gone. Many historians would argue that this relationship between the conquering Arabs and the Persians over a thousand years ago plays a direct role in the ongoing Arabic-Persian, Sunni-Shia conflicts we see in the present day.

Part of the reason for this is the way Islamic expansion took place as opposed to the successor kingdoms to the Western Roman Empire. The way Arabs settled the conquered Kingdoms was basically as a separate army and administrative network. Caliph Umar basically set up a system where the Army and administration took over the Sassanid Persia and Eastern Roman tax systems, often lowering taxes slightly, to pay themselves. They created separate administrative centers such as Mosul(Iraq), Fustat(near Cairo), and Kairouan(Tunisia).

Chris Wickham argues that the division between the Arabs and local populace helped preserve Arab culture and language. Any intermarriage born would be raised Arab. Furthermore anyone who wished to participate in government would eventually have to speak Arab.

Essentially Arab culture mingled far less with the conquered people and that helped it survive far better than European invaders. It also helped that this culture then dominated these areas for a thousand years. The Caliphates didn't face any external threats until the Seljuks and the Crusades around 200 years later.

Source: Chris Wickham "The Inheritance of Rome"