Concerning the HIG, I don't think Apple is eating their own dog food on this one. Software Update pops up on a schedule. As far as I understand it, there is no "app" for it to pop up for when it launches (except for the Software Update app itself).

Also, as indicated here and by others (search for Sparkle on Daring Fireball), some people aren't fond of being interrupted when they launch an app as they launched it to start doing something with that app. I think this may be a case where usage in the field doesn't quite agree with the theory.

Nonetheless, it's a bit of an experimental feature and it's usage is optional so it's up to the dev (or resourceful user - see my comment above) to enable it for their app.

Thanks for the comments, all.