Cory Doctorow relays some comments by Brook Jordan on Privacy Blocker app for Android:
Basically what the app does is scans all the applications you have installed. It identifies what data the apps are requesting about your phone and sending. It then will “fix” the privacy issue by replacing that data inside the app with hard coded (bogus) data. So if an app is sending your phone number back to a server, Privacy Blocker will hard code your number as “55544433333”. You also have the option to override the default values and make it anything you want.
Read more on BoingBoing.
If you’re using Android, you should probably check this app out to see if it will help protect your privacy. There are a number of forums where it is being discussed, and you can also read a review on DroidLife.