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I read that playing back DVDs on (first generation) Powerbook G4s to an external screen can still cause problems over at Doc Searls'. While I thought the problem has been resolved since OS X.1 - at least I successfully played a DVD to an external monitor with it, large screens still seem to be able to cause a problem, according to the report.
Contrary to that account, it seems to me that this problem, or issue as they say, is actually documented by Apple in their Knowledge base article PowerBook: Viewing DVD-Video on External Monitor or TV that is found easily by searching for Powerbook G4 and DVD on Apple's support pages. Although the screenshot in the article is from OS 9, it states hardware restrictions as the cause for the problem - so it should occur regardless of the operating system. The causes they give for DVD playback on an external screen to fail are the video mirroring mode and resolutions greater than 1024×768 pixels, with the latter probably being the one Doc Searls ran into with a 20+ inch display.
So it doesn't look like a big mystery to me. With all the workarounds given, perhaps switching to VLC may be an even better one and remedy this problem as well, along with a couple of other ones that make DVDs suck.
Strangely, an apparently communication-savvy person like Doc Searls doesn't seem to have a feasible way of letting him know about this. There's no obviously visible e-mail contact and the discussion forum seems to require you to get a login name. That sucks.