This is another issue that I can now make public since it won’t be fixed and I guess that VS 2010 RC has it too:
EnvDTE.FontsAndColorsItems.Item(textEditorItem).Background returns 0 instead of actual background color
https://connect.microsoft.com/VisualStudio/feedback/details/518919
This can affect you if your add-in uses RichTextBoxes to colorize code simulating a code window.
I think that VS 2010 uses the correct behavior, while VS 2008 used the convenient behavior…