http://media.cube.ign.com/media/478/478864/vids_1.html
So, although the videos aren't perfect, there are some interesting things to see. Take note of how the Kremlins actually latch on to Mario in the DK level, creating a chain of them that drag behind him, presumably slowing him down.
Mario Tennis is set to hit U.S. store shelves sometime in 2005, officially, but retailers are suggesting it may be bumped up to a November release. With its impending Japanese October release, that doesn't seem too impossible at this point.