I don't really see this happening.
First of all, from a technical standpoint, think of SLI or Crossfire on computers. These multi-rendering technologies use super high bandwidth links between the multiple graphics cards (ie, PCIe to the northbridge and usually a "link" between the two cards). The PS3 simply has a gigabit ethernet port to talk to other PS3s. This is fine for supercomputer applications, but for realtime things like games it's just not suited.
And second, from a financial standpoint, Sony would lose money with such a scheme. They sell each PS3 at a loss, and make up for it with accessories and games. Implementing this idea wouldn't sell more games, expect for a second GT5. But they would lose money from selling another system.
So, it's nice to dream of 16xAA and other eye candy. But it's unrealistic to expect it on the PS3. Sorry!
Edit: My bad, I googled before I wrote this post but couldn't find any multi PS3 applications. But then after this post I found the thing with GT5P at 240 frames per second. Still, this was a tech demo. And because of the financial cost I still think what I wrote above is valid.