The United states has 219,400,000 active cellphones (CIA World Factbook 2007).
I would estimate that a high percentage of these people are using cellphones that come for free with a yearly or two yearly contract from their respective cellular carriers. I suspect every two years the majority of these people renew their contract or move to another carrier. Although data on people tossing cellphones is not available, it would not surprise me to know that 200,000,000 cellphones land up in landfills every two years. I have used my cell phone for about 3.5 years now with no trouble. I just had to spend $5 to get a new OEM li-ion battery on eBay last year and I just picked up another this year. In my situation, I did not need an upgrade because getting a better phone would just not translate into any quantifiable benefit. But there is a definite benefit of reducing your environmental footprint by using your phone as long at it does what you need it to.
Companies like Verizon (I wrote to the Verizon Foundation about 2 months ago about this idea – I have yet to hear from them) can offer their customers the option to renew the contract without a new cellphone and write them a check for the price of the cellphone or even donate the money to charity. I read somewhere that if you really have to be a sustainable business, you need a 360 degree adoption of the concept of sustainable resources. Apparently few companies follow this chain of thought.