Nintendo UK has responded to recent disc issues plaguing Smash Bros. Brawl owners.
"If you have recently purchased Super Smash Bros. Brawl and are experiencing technical problems playing the game on your Wii, please read the following information to learn how this issue can be resolved.
Super Smash Bros. Brawl utilities a double-layer disc which has a large memory capacity. A very small percentage of Wii consoles may have trouble consistently reading data off this large capacity disc if there is some contamination on the lens of the disc drive. Nintendo has specialised cleaning equipment that can resolve this problem. Please do NOT attempt to clean the lens yourself, as you may damage the system."