Off-hand, I'd say that if you are just travelling along the borders of the plane of shadow in order to move quickly, you won't encounter anything (unless an NPC is preparing a carefully planned ambush).
But if you travel through the plane of shadow in order to reach a different plane from your plane of origin, then you might enounter something. (There is an encounter table for the plane of shadow in the 3.5 dmg)
Just my 2 cents.