When you download your own application from the Android Market and then try to change the code and run the new code on your phone you get the following error:
[2010-03-22 11:45:35 - AppXYZ]Re-installation failed due to different application signatures.
[2010-03-22 11:45:35 - AppXYZ]You must perform a full uninstall of the application. WARNING: This will remove the application data!
All you have to do is to go to:
Settings -> Applications -> Manage applications -> select your app -> Uninstall -> OK -> OK
Reverse it true as well, when you try to download from the Market an app that you already installed while development, you will get an error the it is "Incorrectly signed". You have to uninstall the development version.