Eladrin do not sleep. They wait.
That seems to be the general consensus, but I think it's a bit of a biased interpretation of the trance description. Eladrin may be fey, but they're not robots waiting for some event to trigger a reflex reaction!
I don't think the trance description means the eladrin is simply sitting motionless for four hours, waiting for something to happen and ready to jump at the first sign, and getting the full benefits of an extended rest at the end. No, they didn't call it a trance for nothing. (Edit: why didn't they just call it 'Extended Wait' if that's what it is?) The OED defines a trance as, 'a sleeplike or half-conscious state without response to stimuli.'
I think we can agree that you (IRL) have some awareness of your surroundings when dozing (in a sleeplike state), and certainly the half-conscious bit of the definition implies this; but it also says you're without response to stimuli. And what exactly is the eladrin doing when in the trance? I imagine it's a very restful, dreamlike state in which he's still aware of his environment and can readily differentiate between what's happening in his trance state and the real world.
The eladrin may be 'fully aware' when in a trance, but, to me, that doesn't mean 'fully responsive'. If he decides to exit the trance, it shouldn't be instantaneous and should require a bit of effort to shake himself out; he should be a bit sluggish immediately afterwards.
All that said, this is somewhat of a moot thread - I thought the rules for waking from sleep covered a 'sluggish' -5 penalty to actions for a round or two after waking, but I obviously misread - the rules only state that you've a -5 penalty to perception WHEN sleeping to see whether you notice something's amiss and wake up.
I do find it strange that there isn't any penalty to actions immediately following waking up, but wouldn't wish a penalty on eladrin for exiting a trance if the other races don't have one from being woken up!