From StarWars.com:
On Thursday, May 20th, the eve of the 30th anniversary of The Empire Strikes Back, Xbox Live players will be able to customize their avatars into male or female versions of various ESB characters.
Characters include:
AT-AT pilot (male and female)
Boba Fett (male and female)
Bossk (male and female)
C-3PO (male and female)
Chewbacca (male and female)
Han Solo Hoth outfit (male)
Imperial snowtrooper (male and female)
Lando costume with cape (male)
Leia Hoth outfit (female)
Rebel snowspeeder pilot (male and female

Special props include a ballcap with Yoda ears (male and female), lightsaber, and a tauntaun pet.