I'm experiencing the same problem with my one month-old MBP. My MBP supports burning DVD+R DL discs as shown in the System Profiler:
http://i30.tinypic.com/293dmaa.png
Presently, I've tried three different methods of burning a 7.6 GB iDVD project to a DVD+R DL using both HP and Memorex brand DVD-DLs: twice using iDVD, once using Burn, and once using Liquid CD. (For the latter two methods, I was attempting to burn the VIDEO_TS folder that I had created using iDVD.) Burn and Liquid CD both gave me error messages stating that the burn had failed, and iDVD would eject the disc immediately after starting the actual burning process and would just continue to run like nothing happened.
After the burns have failed, the System Profiler shows that my MBP does not support DVD+R DLs:
http://i27.tinypic.com/u0dtt.png
This is very frustrating, especially since there doesn't seem to be a workaround or solution to the issue. Plus, DLs aren't cheep! The only thing I have yet to try is burning the VIDEO_TS folder using Windows software. We'll see what happens.