i'm having similar problems on a brand new iMac 24" and trying the solution that apple offers below, doesn't work:
http://docs.info.apple.com/article.html?artnum=306290
Nor does moving installed themes out of my appication support folder for iDVD.
My application will not stay open long enough even to try to find themes after I dumped them according to apple's instrutions.
Re-installing the appication from the system software disc (iLife 08 came bundled with this new iMac 24")
I've just spend the last 2 hours attempting different solutions --- reloading all of the iLife software, just reloading iDVD, dumping iMovie HD and then downloading the new verion from apple, but nothing is working.
Is it me or did iLife 08 come out of the barn a little too early. iMovie 08 was a real disappointement and now I can't even get iDVD to work. This is not a typical Mac problem. It's usually so easy to use this stuff out of the box. Steve Jobs, please check in on your iLife staff from time to time; they need some help.