This answer assumes that you've got CHDK set up successfully on your camera and can find your way around the menus. The details here are for a Powershot A570, with CHDK 'complete package', which includes the scripts you'll need (installed September 2009).
From the <ALT> main menu on the camera, select 'Scripting Parameters' (you can go there directly by pressing the FUNC.SET button on the camera). In passing, note that towards the bottom of the screen there is a line that says: -Default Script-
Select the top item on the menu - 'Load Script from File...'
Move down to the TEST/ directory and select it (Note that below this there's a line that says 'DEFAULT.BAS' - you'll need this later)
From the list that appears after opening the 'TEST' directory move down to 'BADPIXEL.LUA' and select it. This will take you back to the main Script menu, where you'll see that '-Default Script-' has been replace by '-Create Badpixel.bin-'
Come back out of the menus. You'll see at the bottom left of the camera screen, next to the <ALT> sign, the text 'Create badpixel.com'.
With the camera in 'Shoot' mode, press the shutter button. When I did this I had the camera on a desk with the lens snug up against a book so that the image was black, but I don't know if this is really necessary.
When you press the shutter button, the message:
'Wait Please This takes a few seconds Don't panic'
appears. The camera will take a couple of pictures and do some processing (takes 20-30 seconds) and then say:
'Bad Pixel count 11076' (in my case) 'Press SET to save file or other key to exit'
Then, when you press SET 'File is saved FINISHED'
Now, (IMPORTANT!) go back to the top of this answer and re-read the bit about DEFAULT.BAS. Select this file and your camera is back in normal mode.
When you now go to the 'RAW Parameters' menu you will be able to select 'DNG format' and 'DNG file extension'.