Since the day the Revolution was revealed, Nintendo has been throwing around ideas about putting DVD playback into the system. Originally the Wii was going to be sold alongside an attachment that would allow it to play DVDs, but the idea never came to be. Nintendo then revealed that plans for the DVD compatible Wii are still in works, but instead of a separate attachment it will be delivered in the form of a new Wii console. The console was due to release early this year, but Satoru Iwata announced at the Nintendo stockholder meeting that the new DVD capable Wii was going to be held back due to the still high sales of the current Wii model.

So it seems that the DVD Wii will be released as soon as the current Wii sales start to diminish. Shouldn't really matter to those who currently own one.

Source: Nintendo Holds Back on Wii DVD Playback - WII News from The Game Reviews