Everyone's talking about LBP, and some are wondering if user-created content is even possible on a system with no hard drive.
The answer is yes, and it's been out for months.
Blastworks!
"...overall, this is a great value that provides almost limitless replayability. If you consider yourself a shooter fan you should definitely check it out, but I also recommend it to gamers eager to unleash their creativity and design their own BlastWorks. " - IGN 8.1 overall
It's no LBP by any means, but it shows that user created content exists on Wii, and it definitely works.
http://blastworksdepot.com/games/blastworks/type/level
Sort by "rating" and you'll see some perfect Mario 1-1 levels, N+ levels, F-Zero levels, Sonic levels, and so on.
Of course, if you have a PC you've been enjoying this stuff since you were born. Here's the latest,
http://en.wikipedia.org/wiki/List_of_Half-Life_2_mods