After having a look at a topic in "Digital Image Processing" ForumWhy change in Histogram from Lightroon to Photoshop?
I think it would be a good idea to give the user the choice of the Colorspace for Histogram.
Checkbox with at least the choice between ProPhoto RGB, Adobe RGB and sRGB.
It is important to give this choice because worflows are often different !
Of course you can use a "Default ProPhoto Colorspace" for new Lightroom 3, but please give us the choice !!
Have a Nice Day.