You have to love beta environments, especially those were no help text was installed. I'm starting to play with asp.net 2.0, I even have a pet project to work on as a goal (don't expect it any time soon). First up on the learning list was master pages. Except they didn't work, every time I tried to add a master page to my project Visual Studio died with "Object Reference Not Set To An Instance". You have to love that error message, it should really read "Oi! Lazy programmer! You didn't check for null". Lots of searches later, including a suggestion to delete my entire user profile, the solution finally revealed itself;

Double-click the default registry key for [HKEY_CURRENT_USER\Software\Classes] type REG_SZ.
This bug is caused by having a value set for the key.
Clear the 'Value data' field for this key and click OK.

In my case the default value was xpiFile. Thanks firefox!