Though the Wii seems like the perfect platform for a Mario Paint revival (as does the Nintendo DS), it's doubtful that you'll hear an announcement for that in the next year or so. In the meantime, if you have a copy of The Legend of Zelda: Twilight Princess and are interested in the Wii homebrew scene, you can try out WiiPaint, a new program from Spanish programmer Kontakatilu.
So far, it looks like the version 1.0 application only lets you draw on the screen with a 28-color palette. While that doesn't seem like much functionality, it's a start! Pretty soon*, you'll be looping animations and composing Mega Man 2 themes!
*This probably won't happen "pretty soon."