In the 20th century, cults often times encouraged their members to believe in conspiracy theories. In fact, some of the core beliefs of many cults that were created in the 20th century are conspiracy theories.
In this century, so far very few cults have emerged. In fact, I really only know of two "major" cults that have emerged in this century so far.
The first is called Desteni, which, besides it's very bizarre beliefs, much of which can be read about on the Desteni Cult blog, is that also that many of their beliefs are also based on UFO conspiracy theories and New World Order conspiracy theories, and that they advocate a pseudo-economic system known as the "equal money system". It also encourages people to alter what they think, but how they think, and encourages members of the cult to be very hostile towards critics of the cult.
Besides Desteni, another cult that has emerged in this century, called the Zeitgeist Movement, is based entirely on conspiracy theories, and also advocating a pseudo-economic system known as the "resourced based economy". Plus, like Desteni, Zeitgeist not only encourages people to alter what they think, but how they actually think as well, and it's leaders encourages members to engage their critics in very hostile and volatile manners.
While Desteni it self might be considered stereotypical mind control cult, and Zeitgeist is also considered to be a cult as well, it seems that many conspiracy theorist groups also are cult like in the way they act. Not only do other members encourage follow members to believe what they advocate, but that they also encourage their follow members to distance and isolate themselves from people who might not believe what they believe, and to believe that those people are members of a group that is out to destroy them, and to engage critics in a hostile manner. Also, they are often encouraged to engage in actions that might cause others to distance themselves from them, such as badgering people to believe in what they believe.
Conspiracy theorist groups often times also encourage their members to engage in faulty, or even dangerous medical practices, or reject certain proven medical practices, because they have been told those practices are dangerous.
Also, many conspiracy theorist groups have a kind of "after the end" kind of belief, in which they believe the world will be allot better after these so called "shadow forces" that allegedly control the world are destroyed, and that their own beliefs on how the world should be run is the way to go.
So, are conspiracy theorist groups, cults? Not in all cases, but most do tend to have some cult-like behaviors and actions, and there is the chance such a group could become a cult.
Remember, the Zeitgeist Movement wasn't always considered a cult...