Invalid or malformed application: check Manifest

I started building a brand new Silverlight 3.0 Application today and ran into this error message in my browser the first time I ran my new app:

Invalid or malformed application: check Manifest

I did some research and came across this hint:

In human-readable words, this is what fixed it for me:
1. Open the properties of your project.
2. On the first page, open the drop down list of "Startup object".
3. Select the page your project starts with. In my case, changing the name of the namespace broke the startup object.

from this blog post:
http://forums.silverlight.net/forums/t/11452.aspx

This pushed me in the right direction. I remembered that I had changed my initial namespace in my .cs files, but not in the project properties. I changed the default namespace and then selected the correct startup project, (xxx.app where xxx is my default namespace).

Problem resolved.

Lesson learned:
If your startup object is not correct in the project properties window, you will get the dreaded, "Invalid or malformed application: check Manifest" error.