If I cast {Rending Volley} targeting {Deceiver Exarch}, can my opponent redirect it using {Spellskite}?
Yes and no.
He can use skite's ability to redirect it. However, since skite is not a legal target, it's ability will fail and volley will go back to its original target
6/1/2011: You can activate Spellskite's ability even if Spellskite wouldn't be a legal target for the spell or ability. However, the target of that spell or ability will remain unchanged.
I love that they built the perfect hate card against twin 😄