Have you all forgotten what "Service Packs" are for in the first place? To fix bugs! Over and above "security" patches... like the data-corruption issue that can happen when using hibernation and HDs larger than 128GiB, for example. Just one of many, really.
And as long as you have the "Automatic Update" service set to Disabled, and don't download the SP yourself, I don't see how it could magically end up on your machine. (*)
(*) Interesting tech footnote here: One of the new features in DirectX 8.1 or 9.x, somewhere along the line there, MS added support for something called the "Broadcast Data Architecture". Along with that, if you have capable hardware installed, MS installs a secret network adaptor, that is: 1) Unable to be deleted or disabled by the user, and is normally set to "super-hidden" in the device manager, and 2) silently enabled whenever you enable your TV/video-capture card. I've observed that it stays enabled for the entire windows session, even if you stop your video app.
So it is at least, theoretically, possible that MS could ship things like patches and auto-updates, to properly hardware-equipped systems, using data embedded into the Vblank signals recieved over-the-air, or over a cable connection. Don't forget about the existance of MSNBC, MS's television affiliate, either. I've pulled my tv-capture card from my machine for now, at least.
So really, it's not totally inconcievable that in the future, when we are all running our shiny-new PC '2006 WinXP/Longhorn Media Center Edition PCs on our nice $300 25" LCD HDTV screens, that MS might actually actively broadcast patches and security updates "live" over the air, and we won't be able to block them out, at least not without a lot of bother and possibly a soldering iron besides.
It certainly would reduce the "patch window", the time between when the patch gets released (and subsequently immediately reverse-engineered to discover the exploit), and when the exploits start spreading, relative to how fast the other users' PCs get patched. So that's also an important potentially-useful consideration.