To make your apps not be letterboxed on iPhone 5, you have to re-build and re-submit the app.
So to those of you with a crack-like addition to The Shiny, please enjoy Dali Clock 2.37 in the app store now.
(Yeah, I'll do a new xscreensaver release some time soon, but I've got some bugs there I want to look into first. Dali Clock didn't actually require any code changes, just a re-spin.)