VS 2010 Beta 2 certainly supporting RGB=0,254,0 as background color for command pictures as in previous versions of Visual Studio
FWIW, I have verified that VS 2010 Beta 2 certainly fixes the issue of supporting RGB=0,254,0 for transparent color in 24-bit command pictures that I mentioned here.
So, if your add-in targets VS 2005, 2008 and 2010 you can use that color for the transparent background color of command pictures and they won't appear with lime green in Beta 2 as it happened in Beta 1 or previous CTPs builds.
If your add-in only targets VS 2010 it is preferable to use the new support for 32-bit command bitmaps. I guess that in a few years Microsoft will only support 32-bit bitmaps in Visual Studio.