well you can get an internal Blu-Ray burner for about $700, but i'm not sure OS X would know what to do with it. DVD Studio Pro has built-in support for HD-DVD-sized discs (so in theory an HD-DVD drive would work) but it doesn't mention 50GB discs. I don't know of any internal HD-DVD drives available in the US yet, though (assuming you're in the US... if you happen to be in Japan or thereabouts you'll probably have access to this stuff sooner, if not already)
Either way, it's expensive. If you're just looking to put some home movies on a disc, it's probably better to stick to DVD for the time being, and put an HD H.264 version on the disc for people to play on their computer or something.