I went over the Hoover last September ('07), and the bypass was coming along well, but still had a ways to go. The last parts they need to do are very weather dependent as the winds wreak havoc with cranes and loads, and it's pretty darned high up.
Once the bypass is done, it will give you spectacular views of the dam from way above it, but for sure there will be no stopping for any reason. There has been an ongoing series of shows on the process of this mega project on TV through the past couple of years and it was a thrill to see it in person, even at 30mph!!!