Setup: I have a channel designed to flash-freeze invaders by pumping water up from underground and exposing it to freezing surface air, forming an ice wall on top of an extended bridge. This newly formed 'Ice Wall' often (but not always) fails to kill or even stop sieging goblins, they walk straight through the solid ice tiles.
Trials:
A military dwarf stationed on the bridge died instantly when the ice formed. "has been encased in ice".
A military dwarf stationed next to the bridge, then ordered onto the bridge after the ice formed took one step into the Ice Wall, sharing a tile with solid ice, before realizing his mistake, and flashing '?' as he walked back out, unharmed.
Goblins walking across the bridge as the ice forms may be killed. I would estimate, based on observing a relatively small number of trials, that they are killed approximately 50% of the time.
Goblins walking onto the bridge after the ice has formed are completely ignorant of the solid ice, and pass through the Ice Wall as if it wasn't there.
Removing the bridge, and replacing with constructed floor tiles appeared to give the expected result; Goblins were killed, and could not walk through the Ice Walls.
Additionally, any item in the path of the ice - Goblins, Dwarves, Rocks - was instantly destroyed when the ice froze. This could be the intended behavior, but looking through the known bugs list I saw:
000713 □ [dwarf mode][items] (Report) items can disappear when ice melts after being imbedded
I have not, in this entire period, seen any item, either preexisting or on a creature who was killed, appear in the Ice Wall. When the Ice melts, the tiles are always completely empty.
Addition: It's come to my attention that the items that were frozen, including inventories of the killed units, do still exist. They show up on the stocks screen, and zooming to them takes me to the tile. However, they cannot be seen with 'k', nor can that be retrieved by dwarves, or dumped.