In Oblivion, if you cast a summoned creature or threw a spell from the shadows or laid a trap (you could lay traps with aoe frost spells that lingered), no matter what you would get caught by a guard and the crime would be reported. Any info on whether this detection link has been severed? Because, if not, the silent casting perk in illusion seems a bit useless to me...

I don't think you will be caught this time, with silent casting I think you just need to make sure no one see's you, and cast your spell.

Casting trap runes from hiding is going to be one of my main strategies in Skyrim; I believe, and hope Bethesda wouldn't screw up that strategy.