Source:For the Wii, Nintendo extended the technology to use a full size 12-cm DVD-based disc, enabling it to have the benefits of the Nintendo GameCube Game Disc, while having the standard capacity of a double-layer DVD-ROM. Although the Wii can utilize double-layer DVD-ROM discs, all titles were single-layer up until the release of Super Smash Bros. Brawl. With the release of Super Smash Bros. Brawl, Nintendo has admitted that some Wii systems have trouble reading dual-layer discs due to a dirty laser lens. Nintendo is repairing systems with dual-layer problems free of charge, regardless of warranty status.Spoiler Alert!http://en.wikipedia.org/wiki/Nintendo_optical_discs
What i think about dual-layer problems is that you hear strange buzzing noise when u play ur brawl. So we can send back our wii and get repiared for free!