What is a good clincher/opening statement for a paragraph on cell phones?

This all depends on what you want to say about cell phones.  So, that is the first step--determining an angle, or a focus to write about.  Do you want to write about how cell phones are stunting communications skills?  Do you want to write about how cell phones are an incredible convenience and safety feature for people?  Do you want to write about how using them increases traffic accidents?  The possibilities for a topic are endless.  Once you have that angle that you want to go for, then coming up with a clincher will be easier.

For opening statements, you want something that is catchy, engages the audience, and gets them involved.  You could ask a question; for example, "When is the last time you talked on a cell phone while driving?" Or, "Have you ever texted something to a friend that you would never dare tell them to their face?"  Asking a question will automatically engage the reader, and help them to relate the topic to their own lives.  Another approach is to reference an alarming statistic, or an interesting factoid.  For example, start off by citing what percentage of car accidents are caused by cell phones; you could start by saying "If you are really worried about saving lives, the best way to start is to put your cell phone away while driving," and have that lead into the statistic.

Those are just a couple suggestions--questions, stats, and other ways to engage their interest is a great way to grab the reader's attention when writing a report or essay.  Good luck!