From the contest page:
Fancy yourself a budding game designer? Send us your best work for a chance to win a trip for two to the Nintendo E3 Presentation in L.A. Your task is to design a microgame using a theme of creatures, sports or machines. Entries will be judged on originality, funness, accessibility, humor and adherence to one of these themes.
Best of all, there is no purchase necessary. Entrants who do not own a copy of WarioWare DIY may submit a concept instead of an actual microgame – there is practically no reason not to enter!
Remember the big names that we expect to see at E3 – Zelda, Metroid, a host of others, as well as the 3DS.