Wow, really suprised to see this. I've been pretty much against them including non-nintendo characters, but I think I can make an exception for Solid Snake, as long as they don't go overboard and start throwing in Master Chief or Crash Bandicoot or anything.
Oh, and to me it looked like the Nintendog was just a power-up, the same way a lot of the Pokemon can come onto the screen and start attacking, but who knows. If it were a character it probably would have been introduced like the others.