According to the Windows Mobility Center – Extensibility document, Display Brigthness tile is displayed only when the hardware & driver conditions are met.
Shows the current brightness setting. The slider enables the user to modify the LCD screen brightness. The user clicks the icon to open the Personalization CPL.
Note: In order for the brightness slider to work with your
hardware, you must do one of the following:
BIOS Communication for Display Drivers in Windows Vista (http://go.microsoft.com/fwlink/?LinkId=50987) for more information.
2. Work with your video independent hardware vendor (IHV) to provide brightness support in their video miniport driver.
According to Intel, the 910/915 family will only have Windows XP Driver Model (XPDM) drivers instead of Windows Vista Driver Model (WDDM), which may be in fact the cause of this issue. There was quite a heated discussion on Josh Bancroft's (Intel) blog a while ago and I see there's an online petition to Intel for the WDDM drivers.
Ideally would be for Intel to release the WDDM drivers, which I would rather doubt since 910/915 is an older chipset not used in new computers and probably the pressure that current owners can put will slowly fade away. Alternatively and what I would like to do (although not a C++ programmer, I like to try and learn new things every now and then) is to find out how the Settings Centre manages to change the brightness and write my own tile that will work hopefully not only on R2H but on all Intel 910/915 graphic chips - wouldn't that be glorious, suddenly there'll be mililons of hits on the blog from people looking for their indispensable brightness tile :-) ... mmm... seriously, for now I'll just put this at the end of the long list of would-like-to-do projects I never started :-)