How can I improve the noise cancellation on my Nexus One?
If you are working in a high noise environment, a traditional mic isn't going to help you.  Even with a noise cancelling secondary mic it will be very difficult for the equipment to pick your voice out of the noise and isolate it.  You might look into a throat mic instead, which grabs your voice by direct contact with you rather than conduction through the air.  These are especially suited for noisy environments.