The trick in this, I think, is to get the students to move from being extrinsically motivated to intrinsically motivated. If you can achieve that, then you're an amazing teacher.
In college, or in certain types of humanities courses, I think I could agree. Unfortunately, not all of us teach in an environment where our students care enough to use the cell phone for what it's for.
Also, you have to consider that there are many people out there, myself included, who have to pay extra for every text they send, and who don't have internet access on the phone. You could make it a course requirement, but that would often require making students sign a bigger and more expensive contract for multiple years after your course is complete.
Honestly, I don't know why they can afford one and not the other, but according to most parent-teacher confrences I've held, it is the case. I figure they've got cell phones because it's a more dangerous neighborhood, and because most of the parents work odd hours, but you never know. It also might just be laziness, or pushing the teacher to see how far they can go.
Now what do you do in a math classroom in a low-income district where students claim to be using their cell phone calculators to answer problems, and most of them seem to be doing just that most of the time. I would tell them to put away the phones and head to Wal-mart for a $5 calculator, but I know most of the families can't afford it.