Staying consistent with Wii's inferior processing and graphical specs, official Wii documentation given to development studios confirms the absence of Dolby Digital 5.1 optical outputs. It is likely that Nintendo will continue to use Dolby Pro Logic II for simulated 5.1 audio as it has with the GameCube. IGN writes: "There is a single proprietary audio/video jack on Wii and users have the option to plug in different cables depending on their preference. According to developer documentation, Wii owners will be able to insert a composite, S-video or component audio/video cable into the jack, but all three come standard with stereo left and right channel audio cables."
Though a Dolby Digital solution could still surface being that final Wii tech specs have yet to be announced, don't expect it. Not that Nintendo's target audience will.