I don't think people fully appreciate the SIGNIFICANCE(!!) of both the Final Fantasy and Dragon Quest franchises jumping ship. Allow me to explain...
Not many people in the U.S. care about Dragon Quest, but VIII was the best selling game on PS2 in Japan, and any Dragon Quest game on a system means that system will sell like crazy. The fact that they are bringing Dragon Quest IX to the DS, which already sells like mad in Japan, means they just gave Nintendo a license to print (even more) money.
As for Final Fantasy, it's absurd the number of people who bought a PS1 and PS2 just to play the FF installments on those systems. None of my friends had any interest in the PS2 until FFX was shown off to the public. I would go so far as to say that the FF series has become a console making franchise in the U.S. That the main numbered game in the series might go multi-platform is unheard of, especially since every FF since VII has been on a Sony console.
The other important aspect we need to consider is that it costs a lot of money and takes a lot of resources to develop for the PS3 and 360. It makes more sense to developers, even the ones with lots of money and lots of resources like Square, Capcom, and Konami, to make their new installments multi-platform. It takes less money and resources to port a game than to develop it initially. On the other hand, we see Nintendo with their "underpowered" Wii and DS saying "come develop for us, it's easier and cheaper." The discrepancy in power between handhelds and consoles is getting smaller, such that we will start seeing major franchises have sequels (not spin-offs) on the DS. You can develop and release a DS game in much less time than it takes to develop a console game. They just announced Dragon Quest IX in December and I wouldn't be surprised if it was out by this winter in Japan.