Gizmodo has some great photos of the new Hoover Dam Bypass under construction just down river from the Hoover Dam (or Boulder Dam as it was first known). The $240 million project is set to open in 2010.

From the official Hoover Dam Bypass website:

Construction of the Colorado River Bridge is advancing on the 1,060 foot twin-rib concrete arch. The Colorado River Bridge is the central portion of the Hoover Dam Bypass Project. Construction on the nearly 2,000 foot long bridge began in late January 2005 and the completion of the entire Hoover Dam Bypass Project is expected in Late 2010.  When completed, this signature bridge will span the Black Canyon… connecting the Arizona and Nevada Approach highways nearly 900-feet above the Colorado River.

When I drove over the dam in early 2008 the bridge had barely cleared the edges of the canyon.

The aerial view of the site: