I've never had any problems burning anything on my iMac or Macbook Pro. I burn DL-DVDs, DVDs and CDs regularly on both machines using Disk Utility.
All I can suggest is actually trying a different media, like the message says (ie: try a different brand of DVD), and if that still doesn't work, just call up the Apple store and book an appointment with a Genius and they'll get the ball rolling for you... or just call Tech Support.
Other than that, I dunno what to say dude. Sometimes even a Ferrari needs to go back in to get a faulty part replaced. It happens.
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