At this scale, clusters and supercomputers run into problems with power consumption and heat dissipation, so Exa-scale computing using these approaches is probably many years away. However, there may be a much faster and cheaper path to Exa-scale, using a combination of volunteer computing and graphics processing units (GPUs).
…Could this scenario be realized in the near term, say in 2010? In my opinion, it’s near-certain that GPUs will reach 1 TeraFLOPS by then, and a large percentage of PCs will be available to run BOINC (although the advent of ‘green computing’ will decrease availability somewhat). The hard part will be getting 4 million GPU-equipped volunteered PCs; there are currently about 1 million PCs participating, not all of them GPU-equipped, so an order-of-magnitude increase is needed.
The generally loose coupling between nodes in volunteer grids and the social aspect of such entities make them inappropriate for many classes of problems. But in those cases where they are appropriate, I agree with David that they will probably offer the fastest path to exascale computing. Like Blanche Du Bois, you too can “rely on the kindness of strangers.” Hopefully it will work out better for you than it did for her (yikes).