Via a download code
Activision has announced a special promotion for the upcoming Skylanders Trap Team that will probably excite some Nintendo fans. If you buy the game on the Wii, it comes with a free download code for the Wii U version of the game. This is likely a smart way to capitalize on the insane Wii install base, but encourage parents to upgrade their child's system at some point.
The announcement tags the Wii version's Amazon product page, but there isn't a clear distinction for the code in the item's description -- you can likely take that to mean that every version of the game at every retailer will have the code, but we'll have more info closer to launch.
Skylanders Game [Twitter]