After applying the iDVD 3.0.1 update (via Software Update), I noticed that iDVD will run without a Superdrive present. (I'm not the only one to notice this so it's not just a fluke.)
You still can't burn to an external drive (I tried with my FW DVR-104 drive - burn button isn't enabled), but you can create and edit projects in iDVD 3.0.1 without a superdrive present. (Rather than just refusing to run at startup if no internal DVD-R drive detected.) I wonder if this is intentional or if it will be 'fixed' with the next update to iDVD.