I know that blasts require LoS in 4e and that bursts require line of effect between the target square and the creature using the power but one my players asked if a burst would hit diagonally around a hard corner. My first thought was a hard no, but then remembered reading somewhere about bursts "splashing" around corners decided I needed some clarification. What are the rules for burst attacks and corners?
From my reading of the PHB, both require line of effect from their origin square, and neither require line of sight (p272):
You are probably remembering a rule from 3.5 that allowed spreads to do exactly what you describe. But 4e is a bit simpler; you only check whether there's line of effect from the origin of the spell. (I'm sure there are exceptions, but those will be explicitly called out.)