This was northwest airlines reply to my emailed question about taking my dress on board...
Carry-on luggage will be limited to one piece plus one of the following:
a purse, briefcase, or laptop computer for all flights. A carry-on
piece can have a total linear measurement of up to 45 inches. On all
Northwest jet aircraft, a closet is available in the cabin of the
aircraft to store some items. This closet is relatively small, it is
not full length and a large wedding gown would be compressed in the
closet. The space for this closet is available on a first-come,
first-served basis. Therefore, kindly explain to the agent at your
departure gate the need for you to store your wedding dress on board the
aircraft. If space is available, we will be happy to accommodate your
special request.
Hope this helps a little!