First, I recommend that everyone read the blogs before posting questions that are answered threre. Most of this stuff has been covered and discussed in those blogs. That's that they're there for. The link to the Blogs is just to left of the 'My Apps' link in the top navigation bar.
The DATA Api and the Activities Api ara both discussed there, and will both be available before the MDP Launch date, and I suspect that they'll, at the very least, be in basic beta testing phase before the MySpace DevJams (though, don't quote me on that). MySpaces' big concern is that with the massive number of MySpace users, that both of these functions be fully scalable to.
Also, it might be hard to find because of the absolute slew of posts on the subject, but a couple members of the MDP Team have said that the Activities API will be the first "stage" to implement virality. Other methods will come available as time goes on, but probably not likely before GoLive date. The big concern, as far as that goes, is not preventing Apps from being merely viral to but to keep them from being downright annoying. I think we all believe that we aren't going to exploit auto-messaging features, but, lets face it, there will be MySpace app developers who would, and all that can do is hurt the rest of us who want to make high quality apps that other MySpace users can enjoy.
So, in the meantime, cocentrate on creating the best apps you can, and worry about virality later. :).