My problem with ice-encasement traps is that the ice walls completely obliterate the creatures that have been frozen, items and all. While this can be useful at times, I do prefer being able to take weapons from dead goblins to melt down into goblinite and so on, I would much prefer the items to be stored within the ice until the ice thaws.
An example of how this would work could be as follows: a goblin activates your trap and is encased in a wall of ice. You then send a miner out to mine the wall; upon the wall's destruction the goblin's frozen corpse (which would no longer rot or produce miasma) would pop out, along with the items inside, such as his weaponry/armour. If the frozen corpse is placed in a warm area for a certain amount of time it will thaw out, becoming a "defrosted corpse" which will rot more quickly than a normal corpse. I don't know whether items like weapons should become more brittle having been frozen, but that is possible. The same sort of reaction could happen in a freezing biome on the surface naturally - if a goblin is killed in a snowstorm/blizzard, he could be buried under a few layers of snow. Corpses and items buried under snow could then thaw by the next spring (if it ever gets that warm) and become defrosted/thawed corpses and items.
You can imagine the possibilities here for elaborate Dwarven fridges and the like, with entire stacks of meat products and various corpses being frozen into the ice for future use. I think it would be quite exciting to dig into a layer of ice in a glacier and discover a dead/frozen megabeast from the age of myth, or dead soldiers/civilians from civilisations long-forgotten.
Dwarven Ötzi comes to mind. I also believe that the undead, such as zombies and skeletons and mummies, should be able to survive ice-encasement. If you experience a necromancer siege and a zombie horde in deepest, darkest winter and all the zombies get frozen under the snow, they may defrost by the coming spring.
This would work even better if the ice, instead of becoming an "ice wall", would become an "ice wall" with the object/corpse visibly entrapped. You could even smooth the walls and use them for decoration; imagine smoothed, engraved walls lining the entrance to your fortress, showing the would-be invaders of your fortress trapped within the ice forever, staring back as a warning at any goblin foolish enough to approach.