What might be a possible thesis for an essay on the topic of cellphones?
Science fiction writers such as Aldous Huxley, George Orwell, Ray Bradbury, and Kurt Vonnegut were concerned about the affect that technology would have upon people. (Employing a quote from one of these authors makes for a great motivator [i.e. opener].) One of the concerns of these writers was the increasing dehumanization and invasion of privacy that technology would effect upon societies.
So, regarding the invasion of privacy, perhaps you would like to consider how--to use the words of George Orwell--"Big Brother is watching you." At any moment, someone can take a picture of a credit card, a person or persons in action, tape people's conversations, tap into their conversations, etc. Of course, an effect of this worry about privacy is an increasing paranoia in people.
With respect to the dehumanization of people, one need only look around at a shopping mall or a college campus or anywhere there are many people to notice the lack of interaction among people. (e.g. a daughter and her mother walking through the mall while the daughter text-messages someone rather than talking with her parent, or a van with 3 children each on his/her cell phone.)
Talk with older people who remember the world pre-computerization. They can give you ideas, for sure. Go to the Time and Newsweek or Reader's Digest archives and find editorials and articles.
There are many ways to go on this. The first would be whether or not cell phones should be allowed in schools/ places of business. I think you can provide much in the way of compelling evidence/ analysis about how cell phones are vital to individuals' well being or how they can prove to be a distraction. In terms of the school argument, there is a compelling case for both sides as to why cell phones and their presence in the school environment. Some argue it's a distraction, or they can be used to illegally text or photograph exams/ assessments, or they can be used as a violation of individual rights (such as recording video.) Others can argue that they are vital to maintain contact between students and their parents and can even be used for educational purposes, where students can text correct answers. Certainly, it can be modified to include how cell phones contribute to places of business' and trade secrets or internal memos would replace tests/ assessments.