Installing app from Market when it was previously installed
You could cheat and change the package name for the uploaded app to the market. That way you won't have to instruct your beta users' to do anything differently.

In the future whilst in beta:

 &lt;manifest xmlns:android="http://schemas.android.com/apk/res/android"
  package="com.your.app.beta"


Then once you want to release just do a little refactor:

 &lt;manifest xmlns:android="http://schemas.android.com/apk/res/android"
  package="com.your.app"