Wii games have Dolby Pro Logic II surround sound. This encodes 5.1 surround channels into 2 channels left/right using a "matrix". A decoder can tease them apart and separate them into a decent surround-sound experience. This is different than Dolby Digital or DTS which have separate, discreet audio channels for each.
I have a surround-sound receiver hooked to my Wii. When I switch to my Wii input, I've already programmed my receiver to switch to "Dolby Pro Logic II - Game" mode, and it sounds great. I haven't come across any crazy surround-sound ear-candy in any games, though (stuff shooting by or around you). It's just a general improvement in the sound environment.