I did not even notice this gray/black taskbar until now.
Well, my HP is 5 wk old , still with hardly unchanged default settings (the only change I have done is startup program trimming for quicker start/reboot).
Now, I have become fully aware that taskbar is gray if only one down-sized window is running which is contrasted in dark bar. With full screen sized solo window running, taskbar is black and window is blended in black bar with bright white fonts. No big deal....until you multi-task. Check this out!!!
With multiple running windows, this gray/black transformation proves to be evidently a smart (albeit, nuanced) VISTA feature. When windows are all down-sized, taskbar is gray and only front window is contrasted in black bar that makes white fonts brighter, while all background windows are blended in separate individual gray bar that makes fonts less bright. When one or more windows are full screen sized and one or more are not, taskbar turns black with all background windows in contrasting gray bars with less bright fonts while front window is blended in dark bar with brighter fonts.
Furthermore, when "switch between windows" feature is activated (quick launch)to cascade all running w's (in 3D), taskbar is black and all windows are contrasted in gray bars with faded fonts.
I am convinced it is a very smart default setting. I would not change it a bit for the world.