It has been a long time since Sega did what Nintendon't, but now the two companies are inseperable. After Mario & Sonic held their Olympic games, and Sonic joined the Smash Bros. roster, there has been nothing but love between the two companies. We already know what Sega is bringing in 2009 and it is pretty interesting stuff. They actually are doing what Nintendon't and padding up the empty early 2009 release schedule for the Wii.
Sonic has received a warm welcome on the DS, and is part of history with the Sonic games on the Virtual Console. An actual animated short (Sonic: Night of the Werehog?) was shown on the Nintendo channel for all to see, making people a little more aware of the Blue Bomber again.
One thing we may have not realized (and I'm just yanking your chain here) is what MadWorld could mean for the Wii in the future. As we've seen with the success (it is a success) of No More Heroes, cel-shaded/cartoon games can sell on the Wii. Madworld is just another shot at reinforcing that as true. Bu Sega could also take a second shot....... with the Jet Grind Radio series. Hey, Golden Axe came back (and failed) right? With a little direction and some Ninty supervision, Sega could bring back a super-stylized hit. The possibilities are endless when these two companies work together, and even thought they were the worst of enemies, together, they make an AWESOME team. Team Setendo, Ninsedo, Sedoga, whatever.