about:debug Doesn't do anything on mine...nothing. about:config does indeed bring up a blank screen, and then adds the menu items.
No, I didn't bump a thread to give incorrect information. I added to a thread with information that explained what happened on my N1. Again, about:debug doesn't do anything, about:config added the menu items.
jbhorner thinks about:debug doesn't work because nothing "appears" or "happens" when you click Go. What he doesn't understand is that nothing is supposed to. It just adds new menu items.
For anyone still doubting, reboot your phone, don't type about:config, and instead type about:debug. You'll see the new menu items, which obviously weren't added by about:config since you didn't type it since reboot.
It allows webpages to think you are something other than an Android phone. This allows you to make them think you are running Windows using Firefox, or Mac with Safari... different webpages will show different content to you if they think you are running a mobile platform.