Interesting thread op I must say, when I read the title I was thinking no they dont. But then you mentioned do you need to be able to express pain in order to feel it and I cant help but think no you dont, I guess we're just lucky that plants dont have mouths etc and scream as they are hacked apart to make a salad.
I watched a number of things on slaughter houses, for no other reason that I wanted to know/see how meat makes it into a shop. Im no vegan/vegetarian but I was sort of distressed watching animals being bled out etc... Its horrible :|
They are certainly aware of their surroundings as they will react to stimuli etc and if you damage them they can repair themselves etc. Do they go 'ow that hurt' I dont have a clue, maybe they do we dont really have any way of checking right now.