I'm using Paint.NET 3.0.2580.31 and Visual Studio 2005 SP1 on Windows XP x64 SP1.
When I have a database diagram open in Visual Studio 2005, I right click in the database diagram workspace and select "Copy Diagram to Clipboard". Then I open up Paint.NET and go to Edit, but "Paste in to New Image/Layer" is grayed out.
Interestingly enough, when I open paint (windows paint, not .net) I can past there and then copy, and then I can "Paste in to New Image" in Paint.Net.
Maybe a bug?? Other than that, I must say I love Paint.Net, great work!