The Role of Lying in Islam
The following is excerpted from an article on the answering-islam.org website.

Muhammad clearly condoned dishonesty, as well as murder, when it suited his purposes.
Lying today
Imam Abu Hammid Ghazali says: "Speaking is a means to achieve objectives. If a praiseworthy aim is attainable through both telling the truth and lying, it is unlawful to accomplish through lying because there is no need for it. When it is possible to achieve such an aim by lying but not by telling the truth, it is permissible to lie if attaining the goal is permissible." (Ahmad ibn Naqib al-Misri, The Reliance of the Traveller, translated by Nuh Ha Mim Keller, amana publications, 1997, section r8.2, page 745)
Note that Al-Ghazali is one of the most famous and respected Muslim theologians of all time.
Articles which are exploring this topic further:
Understanding Dishonesty and Deceit in Islam
Is Lying a Sin, or Just a Good Strategy?
One of the foundational theological problems for Islam in this matter is the fact that Allah prides himself to be "the best of deceivers" (Āl 'Imran 3:54, Al-Anfal 8:30). How then can deception be negative and morally wrong for Muslims when it is a matter of pride for Allah, the god of Islam? For a detailed discussion of this matter, see Allah – The Greatest Deceiver of them
All and further articles listed under DECEPTION.
One specific instance of a Muslim missionary using deception to make Islam look more acceptable to a Western audience: Sunni Muslims and Taqiyyah.
One should always be very cautious and careful regarding debating Muslims, as many see no problem to use lies and deception on those occasions. Oftentimes, they do not understand dialogue and debate meetings as an open and honest exchange of convictions, but it is part of the Islamic war against unbelievers in another form, and "war is deceit" as Muhammad said. A couple of examples are given in these articles:
Nadir Ahmed: Taqiyya Incarnate
Jesus as God's Son in Luke-Acts: Responding to the Distortions of a Muslim Taqiyyist
And deception (a.k.a. the wise choice of words) also features in the high-profile "dialogue" initiated by a letter of 138 Muslim scholars to the leaders of all Christian churches, see the detailed analysis The Truth About "A Common Word" and other material provided in our overview page on "A Common Word Between Us And You".
For further implications and current reality, see e.g.
The Islamic Agenda And Its Blueprints
The Peace of Jerusalem and Yasser Arafat
Islamic Law On Female Circumcision
Civilization of Deception
Two articles giving details regarding a particularly blatant case of Muslim lies for political expediency:
The Muslim Claim to Jerusalem
The Modern Destruction of the Temple Mount (this is criminal!)
One particular deception that resulted in great destruction and the killing of innocent people:
The Muhammad Cartoon Hoax (for further discussion of the cartoon issue, see Is Islam above Criticism?)
Sunni Muslims often criticize the Shi'a for their use of Taqiyya, or not showing their faith openly. In other words, if a Shi'a Muslim's life is in danger, he may lie as long as he holds true to Ali [sic] in his heart.
Related:
Muslim (and other) Hoaxes