I couldn't figure out any possible way to align buttons in iDVD 4 and then I ran across iDVD Companion on Apple's website. It's an AppleScript app which you launch and use along with iDVD. It appears to have been written for iDVD 2.1. In addition to aligning buttons, iDVD Companion allows you to "nudge" buttons and text in very small increments, and exactly position Title text on the screen. I have not tested all the functionality with iDVD 4, but the button alignment works perfectly.
The instructions are very easy, so just download it and get started!
[robg adds: Perhaps even more interesting to those who are interested in learning AppleScript, Apple has also posted the source [1.1mb download] for the entire AppleScript Studio project...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040209171750224