According to CHUD, it will be announced in the coming weeks that Billy Crudup (Almost Famous) will be cast in Zack Snyder's Watchmen as Dr. Manhattan:

A very reliable source tells me that Snyder has settled on Crudup for the role of Watchmen's only super-powered character, a scientist who accidentally gets annihilated on an atomic level and reconstructs himself into a big, naked, blue, floating demigod.
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The site claims there have been a number of names going around in consideration for Dr. Manhattan, including Keanu Reeves and Jason Patric.

Watchmen arrives in theatres in 2009.