Outside of the occasional Wii port like Transformers: Cybertron Adventures, Next Level Games has been working with Nintendo quite a bit since their inception. Not only are they responsible for Super Mario Strikers, but the new Punch Out!! and most recently, Luigi's Mansion: Dark Moon. They're not owned by Nintendo, but according to co-founder Jason Carr -- it's time to make their partnership official, and for right now, Next Level Games will develop exclusively for the Big N.
Next Level Games consists of under 70 people, and given how great the relationship with Nintendo has been, they want to keep the good times rolling. Speaking on this, Carr states, "We have worked with a lot of other companies before, and we're super happy with our relationship with Nintendo. There's no reason to look anywhere else. They keep giving us better and better IP to work with, and as long as we do our job and make good games for them there's no reason for us to venture out."
It makes sense to me! Next Level is a great developer, so I can't wait to see what they handle next. There's also no word on them actually getting acquired by Nintendo either, so they have a lot of leeway in the future.