When opening my blog today I received an error message indicating that the WPF/E (Silverlight) version installed on my system is out of date. The result of the error is that my post about Silverlight isn't working anymore (in fact, the area of the app is not refreshed and shows bits and pieces of other content on the page).
The key to get it working again is to update the project with the new 'Silverlight Tools Alpha Refresh for Visual Studio', which is for VS2008 Beta II. But than I ran into error 'AG_E_INVALID_ARGUMENT 2210 Error' when the app tried to load my button (which is in another assembly). To solve this problem you have to make sure that the build action of the XAML control is set as 'Embedded Resource', which is described in a blog post of Rob Conery.
So the browser requires you to upgrade the client machine, but the software isn't backwards compatible. I guess you shouldn't write software with beta's if you don't want this to happen...