It looks like I am having the same problem. My situation is as follows:
iMovie3 is started, iDVD3 is not started. I edited a movie in iMovie and chose to Export to iDVD. I message appears saying "Launching iDVD" and there we are. Nothing has ever happened since, at least it seems so. The idea - if I understand this message right - would be to have iDVD started up automatically.
So, here is what I found out:
1) My movie was also about 90 minutes long. So I tried to Export a movie that is only 1 minute long. The same problem comes up: "Launching iDVD" and nothing happens. So, after a while I closed iMovie. What I noticed was that I had another file in my folder where the iMovie project was - same title as my iMovie project but with the extension .mov. Then, I started iDVD manually. I first tried to drag and drop the 90 minute .mov file onto the iDVD pane but that didn't work. In fact when I tried to import it through the menu, I received a message that this file was not compatible. Afterwards I tried to drag and drop the one minute .mov file and it worked. After that I could click on the Status tab in iDVD and see that the movie was encoded as I was used to. Now, what happened? I suspect that my Mac was indeed exporting the file (in the background). The problem is that you don't have a clue what progress has been made and a 90 minute file may take rather long. The way iMovie2 and iDVD2 worked was that you export the movie and would see a progress bar. After the export was finished you would drag and drop the created file into iDVD. Also, you could give the exported file a name other than it seems to work with iMovie3/iDVD3. I am quite a bit frustrated how the new versions handle the export. What is weird though is that the 90 minute .mov file could not be imported into iDVD but I can still watch it in Quicktime! So the solution seems to be: Be very patient and after some hours just to try to import the created .mov file into iDVD. - No guarantee though!
2) I happened to find a possible workaround for this problem. The export-to-iDVD function does not work with iMovie3 and iDVD2. When I was looking on the Apple support website I found a help for this problem: The topic is called "How to Export to iDVD 2" and this is what is suggested:
1. Choose File > Export.
2. Choose "To QuickTime" from the Export pop-up menu in the Export window.
3. Choose "Full Quality DV" from the Formats pop-up menu.
4. Click Export.
5. Type a name for the exported movie and click Save.
I guess this should work with iMovie3 and iDVD3 as well. As I am typing this, my Mac is exporting the 90 minute movie. Hope it works. The chapter marks will not work though!
If there is anyone out there not having a problem with the iMovie3 export to iDVD3 export, I would certainly be interested in knowing how it should work properly.
