Well, there certainly won't be many multi platform games for the revolution, but I think lots of third party developers will put out games for it, because they can do stuff on the revolution that they can't do anywhere else. But, of course, when I think like this the DS comes to mind. Similar scenario, but where's the third party support? There are very few decent DS games that aren't first or second party titles. But, there are some 3rd party titles for the DS on the horizon that look good (Lost in Blue, Castlevania, Trauma Center, and maybe a few more that I'm forgetting.). Anyway... what was I talking about? Oh right, I don't think there will be many multi platform hits on the revolution, but I think we'll see lots of third party guys at least experimenting on the revolution with original IPs, or maybe some oldies.
But still, I don't think any real gamers would choose the revolution over the PS3 or 360 if they could only get one next gen console. Is mario and zelda worth missing out on great multiplatform hits? I certainly will get one, but I'm also getting a 360. But anyway, who knows what will happen.
As for Nintendo going third party. I can't imagine that ever happening. Even if the revolution bombed, they still make so much money from the portable market that they'll just keep going, and because they're very, very stubborn. /IMO