How to delete an app installed with Amazon Appstore? I've installed an app using Amazon Appstore, but I don't see a way to uninstall it. Amazon support has failed to respond to my question after 2 days.

I prefer a method that does not involve deleting it with a file manager or some sort of root access.
From the homescreen hit the menu button and select Settings.  Then select Applications, and then Manage applications.  Find the app you want to uninstall and select it, then clear the data associated with it by tapping Clear data and then tap Uninstall.

Voila!

Edit: I actually just noticed this in Amazon's official help documentation for their Appstore (from June 14th, 2011):


  ...  
  
  Amazon Tested
  To ensure the highest
  quality apps in our catalog, we take
  all submissions through a series of
  tests. In this process we screen the
  apps for known viruses and malware,
  test the app to check that it actually
  does what it claims to do, and test
  whether the app affects the stability
  of the other functions of your phone.
  These tests include (but are not
  limited to):
  
  
  Installation: The application installs
  and opens, upgrades can be installed
  and the application can be uninstalled
  by following the phone's regular
  uninstall method.
  
  
  ...


(Italics added by me.)

Clearly they intentionally left out an Amazon Appstore-native method of removing apps you've downloaded and installed from them.