Donald Sensing, analyzing the gunpoint conversions of the FOX News journalists who were kidnapped in Gaza and then subsequently released, comes to the conclusion that Muslims necessarily accept a forced conversion as valid:
So according to the precepts of Islam, Centanni's and Wiig's confessions were completely valid. Any Muslim, not just their captors, considers it so. That they were uttered "at gunpoint" is unobjectionable. The guns simply enabled the two newsmen to understand that submission to Allah was required of them. Regardless of what Centanni or Wiig may think or believe, Muslims now consider them to be of their religion.
Unfortunately, this interpretation of Islam is driven as much by Sensing's own Christian polemical agenda as it is by any real attempt at understanding the issue of conversions. It is rooted in a false understanding of what the "submission" that is Islam's name means - a deliberate misunderstanding to be sure. It is just as false as a muslim claiming that Christians are polytheists because of the Trinity doctrine.
The truth is that the acceptance of Islam is a purely individual decision. The Shahada (There is no God but God and Mohammed Is His Prophet) is an oath. Sensing portrays this as a submission of slave to master, but in reality it is no different from the oath of citizenship that immigrants take when becoming US citizens.
Whether or not a person is muslim depends solely on whether that person declares it to be so. Since Centanni or Wiig appear to have indicated that they only gave the shahada out of fear for their lives, it's essentially obvious that they are not truly muslim.
This issue also touches on the issues of takfir and of taqqiya. I will leave discussion of the latter to Ali Eteraz (link forthcoming).
UPDATE: a commentator to Sensing's post observes,
the original post did not cite legal opinions, which is what is relevant. It would be as if Rev Sensing tried to guess what Orthodox Jews could eat, by reading Leviticus and speculating, instead of actually consulting a source on Kashrut. Wed get halacha according the school of Sensing, not anything that told us anything about what ANY Jews actually do. Somehow we accept this kind of reasoning wrt Islam, cause of the manifold problems Islam has - and yes, Im quite aware of who and what our enemies are. I also think the threat from Salafist terrorism, and the related threats from Khomeinism and pan-arab nationalism, are serious enough to want to be precise in our investigations of the Islamic world.
Sensing did not cite legal opinions of Islamic jurists nor did he even bother to cite the Qur'an! (I prefaced this post with one verse, myself). I will also point to this court case from Cairo back in February 2006:
Cairo, Egypt - Two young Coptic Christian women whose father had converted to Islam when they were infants have won a court battle in Egypt to retain their official religious identity as Christians.
Now 18 and 19 years old, Iman and Olfat Malak Ayet will be issued national identity cards matching their Christian birth certificates tomorrow.
In the final verdict, presiding Judge Farouk Ali Abdel Kader of Cairo’s District No. 1 Administrative Court declared that the civil authorities had conducted a “non-justified intervention” by imposing upon the two plaintiffs a belief they had not chosen.
Nearly three years ago, the Ayet sisters were surprised to learn that, before his death, their father had changed their identities from Christian to Muslim on government records. The change also left them with new Muslim names.
This is just a single example, obtained from a casual google search, but it serves to rebut Sensing's assertion that the "precepts of Islam" compel muslims to accept forced conversions as valid.