I did not notice this before, but now that I've seen it, it's undeniable. At Nintendo's E3 Conference, they played a short video of Super Smash Bros. Brawl. At the very end of it, you see Donkey Kong jump down onto Yoshi's stage. You can view the video here. That makes the video even better for me. I have a theory that the video footage we saw is part of a trailer that we're going to see later at E3, where they'll officially announce DK and maybe more. Thanks to the people at the Nintendo Forums for pointing that out for me!
UPDATE 2: I had a screencap of the YouTube video before, but here is a better picture: