Solo's cuffed before being lowered in, but the cuffs are removed I believe, by either the Ugnaughts or Stormtroopers, but I'm pretty sure someone uncuffs him.
What he still has on are his bicep restraints. The one and only theory I've heard about how/why those miraculously disappear, is that they "dissolve" in Carbon Freezing, and are just to keep his arms from moving too much. Take that with a grain of salt, it was someone's theory, and it worked well enough to me as to why the bicep restraints disappeared.
I believe though, he is uncuffed prior to being lowered in.