If you don't absolutely need your laptop, leave it home. Like everyone else has said, everyone is out to make a buck. You'll see examples of the poverty just outside of Cancun (and inside) on your way to your hotel.
Use the buses. They run up and down the main drag at all hours of the day and night, and will not hesitate to pull over and pick you up if you flag them down. A fare is something like 6.5 pesos (60cents), and I never felt uncomfortable on them, even alone at night.
Bottom line is just watch yourself and others in your group. Don't do anything stupid like try to buy drugs or public urination. They really focus on these things, especially with americans, because they know you will try to pay them off, or demand you pay them off, to avoid legal trouble.
Although I am glad I had the experience of visiting, I am in no hurry to go back to Cancun any time soon. I just have a hard time letting my guard down in environments such as these, and have a hard time relaxing when I feel the need to constantly watch my back.