Basically, they're adding a north wing to the part of the museum that lies east of the railroad tracks. The expansion is also intended to help facilitate the flow of traffic within the building and to improve access for people with disabilities. They're also planning to relate it to Millennium Park with a brand new main entrance on Monroe.
Here's Piano's description of the new wing, taken from his website:
Um, did he say "umbrella (flying carpet)"? I'll be damned if I can figure out what he's talking about. An umbrella idea I get but a flying carpet? Kick ass for him if he can pull it off.
The new building will be located in the north-east quadrant of the Art institute site, at the corner of Monroe St and Columbus Avenue. It will be be a light glass, steel and limestone walls structure, which will fit perfectly into the 19th century architectural identity of the existing buildings.
This 230,000 square feet total structure (three floors above ground and one floor underground) will provide 63,000 square feet gallery of modern and contemporary art galleries. It will also provide new public functions: large educational facilities, a museum shop and a café located at street level. Underground will take place storage and various handling areas.
The building will be organized along a top lit internal street which will connect visually and physically the Art Institute to the neighboring Lakefront Millennium Gardens and their 10'000 seat outdoor amphitheater. This top lit 300 feet long internal street will create a new major north/south axis of circulation in the Art Institute.
The existing east-west axis of circulation will be improved by the remodeling of Gunsaulus Hall. This 19th century steel structure will be unclad and partially glazed in order to reveal its historical identity and to allow dramatic views to the outside.The new building will be protected by a 216-foot wide, square luminous sun-shading structure, like an umbrella floating over the upper floor galleries. This umbrella (flying carpet) will also protect the new south garden in order to create an outdoor sculpture gallery.
One of the most disappointing missed architectural opportunities in this city was the Museum of Contemporary Art's uninspiring choice of building design, so I would love to see Piano pull off something this creative and thoughtful at the Art Institute. I also like how his vision seems to respect and build on the existing nineteenth century building. It speaks quietly with an imaginative, light-oriented response to the stateliness of the current structure. (It also definitely conjures thoughts of Pei's glass pyramid response to the Louvre.)
Anyway, here is the link for the photos of the new wing planned by Piano. He even includes close-ups of the "flying carpet," but they don't help me get it. Maybe you can figure it out. Either way, stay tuned. An exhibit of Piano's work opens on May 31 as well at the Art Institute.
--Art Institute Press Release from 2003 with background, overview