Though the exact dates have not yet been set In mid to late-October 2011 the Alaskan Way Viaduct will close for nine days. This, from the Washington State Department of Transportation.
That means you can expect times of gridlock traffic as drivers attempt to get north or south through Seattle.

The purpose of the closure is to permit demolition and construction work to be completed on what WSDOT refers to a the "southern mile" of the viaduct including:

Demolishing approximately 300 yards of the viaduct near S. Royal Brougham Way.

Completing a four-lane construction bypass to connect the central waterfront section of the viaduct to the north end of the new SR 99 bridge.

Connecting SR 99 lanes south of S. Holgate Street to the south end of the new roadway bridge.

Completion of construction of a new southbound off-ramp to S. Atlantic Street. The existing ramp to S. Royal Brougham Way will close.

After the nine-day closure and completion of the work both north and southbound traffic will travel on the first of two new bridges currently being built to the west of the existing viaduct. The second new bridge is scheduled to open to traffic in 2013.

Drivers will pass through a large bypass located between S. King Street and S. Royal Brougham Way. The bypass will connect the new SR 99 bridge to the existing viaduct at S. King Street. It also keeps traffic on SR 99 moving while giving central waterfront viaduct replacement crews enough space to work.

South of S. King Street, SR 99 will have two lanes in each direction. North of S. King Street the viaduct will remain in its current configuration with three lanes in each direction.

The speed limit on the majority of the viaduct will be reduced to 40 mph. The construction bypass will have an advisory speed limit of 25 mph.