They could have done a little better describing it, but I think you are putting too much into the literal mechanics.
To me, even if it is dim light you can't Stealth right next to a guy (unless that guys can't see you or is "distracted"). You technically have "concealment" but I think that is pushing it in a combat. If you disallow Stealthing purely in dim light alone, I suspect a lot of your problems will disappear.
You can Stealth behind a bush that gives concealment though because it is physically obscuring you. As long as you stay in or behind the bushes relative to your target, you are maintaining your concealment.
You can Stealth behind a low wall that gives cover, because it is physically obscuring you. As long as you stay behind the wall relative to the person you are Stealthing against, you are maintaining cover.
Standing next to a guy in dim lighting is concealment, but it is not "unblocked line of sight" to me, even if the target has only normal vision. Fog and effects like that, maybe, if you move far enough away for them to actually lose visual contact.
You could use the concealment from dim light to sneak up before a combat begins, thereby getting you a surprise round + Combat Advantage, but once combat starts everyone is paying total attention.
A monster can definitely apply intelligence to a situation as I was saying in my last post. Just as you would expect a player to in a similar situation. His mind is not somehow fogged by a successful check, he just can't pick you up with his 5 senses.
If you run and hide behind cover or in a square with concealment, they saw you go run into that area before they lost track of you. He may suspect very strongly you are in those bushes, but he isn't seeing you and doesn't know for a fact that you are actually there. There's still the chance you ran behind the bushes and teleported after all -- Eladrin Rogues could have a lot of fun that way.
If you were to attack them before they actually beat your Stealth check with a Perception though, you still get Combat Advantage even if they "suspect" you are where you actually are. See the Combat Advantage section, it says when a defender is unable to see the attacker.
Once you attack, you no longer count as being Stealthed. You will have to try again. At the very least, in that instant you were not Stealthed, the target(s) would know where the attack came from, so it is probably to your benefit to move a little bit.