The little problem that I reported when using the SetTabPicture method with a 32-bit bitmap with alpha channel for transparency has been fixed according to the last notification from Microsoft for the following VS 2010 beta build:

EnvDTE.Window.SetTabPicture doesn’t support 32-bit bitmaps with transparency in alpha channel when toolwindow not active
https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=470236

That means that for the first time a Visual Studio version (2010) will support 32-bit bitmap with alpha channel for transparency both for add-in command pictures (EnvDTE.DTE.AddNamedCommand) and toolwindow pictures (EnvDTE.Window.SetTabPicture), and you can get rid of magenta (RGB=255,0,255) and almost green (RGB=0,254,0) hacks in add-ins, which is really great and very close to the support for icons with built-in transparency that I have been battling for all these years. I don’t care if I have to use 32-bit bitmaps with alpha channel instead of icons. It is also great because in most cases the toolwindow picture is the same than the picture of the command that shows the toolwindow, so you can use the same picture without re-colorizing the background from green to magenta or transparent when calling SetTabPicture, or providing two versions of the same picture with different backgrounds.

The only problem is if you still have to support old versions of Visual Studio (2005, 2008) which I guess will be the case for most commercial add-ins because VS 2010 doesn’t support the RGB=0,254,0 color for command picture transparency that was used in those versions, this issue is still active at the time of this writing (Update (August 6): VS 2010 Beta 2 will support RGB=0,254,0 too for backwards compatibility):

Custom pictures in CommandBarButtons created by add-in don’t show a transparent background
https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=461192