From what I understand to put the game in widescreen you need Black & White patched to 1.2 or Creature Isle and to open up regedit, go to HKEY_CURRENT_USER\Software\Lionhead Studios Ltd\Black & White\BWSetup and change ScreenH and ScreenW.  The only problem is that when I get there, ScreenH and ScreenW don't exist.  I tried opening up both Black & White and Creature Isle, and restarting the computer, but the values don't appear.

My computer is running Windows Vista Home Premium.

First run the game and see if you can see v1.20 in the bottom left of the loading screen.
If if you do then you are able to edit.
If not install SP1 and re patch just with  v1.20
Next run setup from the main game folder this  will set the default values, set all on max exept screen size leave that at any lowish value. Save and exit.
Now you will be able to edit Screenw and ScreenH in the registry.
.
Important bits are run setup to establish values.
Sp1 allows Vista to see the patched game properly.
You may not need to do all the steps but  if you do it will work.

I already have SP1 installed, and the game says v1.20 in the bottom right corner..

I ran the setup in the game folder, and it worked.