A few weeks ago, Apple released iDVD 3.0.1 Update, an apparently minor patch that provides, in Apple's brief description, "improved performance and stability for encoding, burning, and managing your iDVD projects." (The updater is a 4.1 MB download.)
<http://www.apple.com/idvd/download/>
What Apple didn't mention at the time, but recently revealed in a Knowledge Base article, is that the update makes it possible to run iDVD 3 on any Macintosh with a PowerPC G4 or G5 processor - not just Macs with a built-in SuperDrive, as previously required. Of course, you won't be able to burn the final DVD disc on a non-SuperDrive Mac, but you can build projects and encode them in preparation for burning on another machine.
<http://docs.info.apple.com/article.html? artnum=93274>
Unfortunately, you can't purchase a copy of iLife and install it on your Mac; the installer won't install iDVD 3 without a SuperDrive present. (I wasn't able to test to see if a recent boxed copy of iLife includes iDVD 3.0.1, and whether that would install on any Mac; since its introduction, the iLife package has been updated to include iTunes 4. However, I doubt that copies with iDVD 3.0.1 would have made it into the retail channel yet.) Instead, you must copy the iDVD 3 application folder from another SuperDrive-equipped Mac, then run the iDVD 3.0.1 update to make it work.