Just announced at the OPX website.

"Official Pix is proud to announce Carrie Fisher (Princess Leia) with our first round of guest announcements for Celebration IV! Carrie will be joined by many fan favorites and many first time Celebration signers including: Kenny Baker: R2-D2, Lightning Bear: Scout Trooper, Stormtrooper, Paul Blake: Greedo, Richard Bonehill: Snow/Stormtrooper, Mike Carter: Bib Fortuna, Mike Edmonds: Logray, Ugnaught, Garrick Hagon: Biggs Darklighter, Gerald Home: Tessek, Mon Calamari Officier, Richard LeParmentier: Admiral Motti, Dave Prowse: Darth Vader, and Tim Rose: Admiral Ackbar."

Also of interest is this page at the OPX website which reveals how many coupons are required for each celebrity autograph. Plus, look at how there are two sections, "Guests Of Honor" & "Special Guests". It will be interesting to see how these two sections fill up!