What about beings that are not human (aliens maybe, or even animals)?
I am imagining the situation of either sitting and doing something (posting on sciforums) and feeling like I'm being watched while I'm doing this. If it is a human presense of conciousness, then I think you're right, it would depend on the relation, but I think this is based on a more fundamental issue: motivation, or, at least, the ability I have to guess at the motivation. If some non-human entity were watching me (not a pet, but something that could prove threatening), then I don't think I would have any idea about the motivation. That would really creep me out.
The other situation is walking around a dark corner or something like this. Again, the most fundamental issue, I think, arises in the case of a non-human entity. How would this entity react in such an abrupt situation. But, it's not the encounter that I'm asking about, it is the feeling that such an encounter is immenent that has the real potential to send those chills up and down the spine. The point is, nothing has to happen to make these creepy situations. (And really, I should say, "consider these two situations in the region of time before anything else happens. In these regions of anticipation, which anticipation sends more chills.")
So, with this in mind, which is creepier: the "being watched" or the "abrupt encounter" anticipation.