GM does not require UDID registration. However, it ABSOLUTELY requires iTunes 9.2, which is only available to developers, and only for Mac. If you ain't got it, your phone isn't getting activated, even if you somehow get the OS installed.
Further, in my experience (only 5 phones, granted), only 3GS can be "upgraded" with 4.0 GM. I've tried upgrading three different 3G phones, and none of them would take--it's full restore or no go. And following a restore, of course, all your data is gone, and has to be re-synced--which, in turn, can only be done from a Mac with iTunes 9.2. The phone won't even show up in 9.1.1, let alone allow you to sync or transfer music.