The question sounds so obvious that Captain Obvious may swoop down and punch me in the head, but hearing it straight from Nintendo would be better. While this isn’t confirmation, we do have Nintendo’s Shigeru Miyamoto talking about how they want to continue to allow gamers to use their Miis on other game systems going foward.
IGN asked about Mii avatars on the 3DS, pointing out that we’ve already seen Mii games on the 3D portable. They asked if there would be a way to transition our Mii characters from the Wii to the 3DS. Miyamoto beats around the bush a bit, as you’ll see:
We need to keep in mind how we evolve the Miis and how we think about compatibility with Miis between different systems, but that’s something that we’re definitely giving consideration to and want to continue to try to allow people to use their Miis on different Nintendo systems going forward.
I think there’s a pretty good chance we’ll see Miis on the system, though I’m not sure how you’ll get that Mii data from the Wii, or if Nintendo will even want you to bother with having the same “character” persist.