What are the population level of Feywild creatures in Nentir Vale? It must not be massive. For example, everyone should be terrified of scarecrows.
My opinion is that the feywild is probably the most common source of creatures after the wild. Gnomes, elves and eladrin are all races closly linked to the feywild, so travel between the two is more common than any other 'plane' (e.g. Shadowfell, Astral Sea etc).
The Nentir Vale is getting a gazeteer early 2011 I believe, perhaps this will give you a better answer, but at the end of the day it can be anything you want.
If you want all encounters to be fey creatures then you simply say that there is a portal somewhere that has created a large opening and a flood of fey creature have come through, decimating the indiginous population of natural creatures (bears, wolves etc). You could even turn this into an adventure (close the portal, restore natural balance etc).