FCP is a very good program, hell I've been living off it for two years now, but like everything there can still be improvements. People probably were saying the same thing about Premiere before FCP. iDVD is a good consumer app, and you can't beat the price, but try to recreate some of the menus and actions of better DVD releases from Hollywood and you will immediately see it's shortcomings. ditto for DVD Studio Pro. DVDSP has some serious limitations right now, especially for the pro user, but once again it's hard to beat for the price. (A single feature upgrade on my sonic system costs three times what DVDSP costs.)
Adobe can build a DVD system that will compete and may even be better than the current version of DVDSP, but it will be a hard fight. Beat iDVD? Why bother competing against a free app? That only leaves two markets...the mid range Pro DVD authoring arena for $1000-$10,000 on both platforms, and the big market of consumer/prosumer PC users. And now that you can get an IDE DVD burner for $400 or less that market will grow exponentialy. My guess is we won't see an Adobe DVD app for the Mac, but that's just my guess.