When Imoen should be stuck in spellhold, she is actually standing at the same place she was first taken away by the cowled wizards.
I think this will be fixed in the new versions.
This still happens with the latest git version of gemrb as of 29 January 2013.
Save is here: http://mir.cr/1PQWETLM
this was with a new game (started with the git version)?
Yes, what happens is that Imoen disappears after the cut scenes, then if you save the game and reload it again, she reappears- that's what happened to me.
this was fixed again recently.