It won't be available as an add-on attachment, though Nintendo expect to release launch details later this year...

A reader at GoNintendo recently emailed Nintendo regarding future versions of the Wii including the DVD playback feature.

The reader was sent this response:

... Nintendo and Sonic Solutions have made an agreement to allow Nintendo the use of Sonic Solutions' DVD technology in a future version of the Wii console. However, launch details, like where (regions or countries) it will sell, release date, or price (although it will be more expensive than the original version) are currently unavailable and won't be revealed until sometime in the latter half of 2007. It will not be available as an add-on attachment.

For those who already own a DVD player at home, the launch version of Wii will be the best choice. This version of the console is more affordable and you'll be able to enjoy Wii games immediately.

So, Nintendo's answer is that a DVD-Wii is on the way, but it will not be available for ages, so you may as well buy a Wii now. Nice answer, Nintendo!