If you're looking for a TweakUI replacement for Windows Vista, there are several contenders, but the one that's closest in spirit and functionality is, I believe, WinBubble, a freeware application by Lawrence Albert. You can find out more about WinBubble on its Web site, but I'm especially amused by its ability to change the OEM information and logo, which you can of course populate with your own information. Microsoft may never release TweakUI for Vista. But they don't have to: WinBubble is what you're looking for.

Thanks to Lawrence A. for the suggestion.