2013-04-15 : Because a spell with overload doesn't target when its overload cost is paid, it may affect permanents with hexproof or with protection from the appropriate color.

2013-04-15 : Casting a spell with overload doesn't change that spell's mana cost. You just pay the overload cost instead.

2013-04-15 : Effects that cause you to pay more or less for a spell will cause you to pay that much more or less while casting it for its overload cost, too.

2013-04-15 : If you are instructed to cast a spell with overload “without paying its mana cost,” you can't choose to pay its overload cost instead.

2013-04-15 : If you don't pay the overload cost of a spell, that spell will have a single target. If you pay the overload cost, the spell won't have any targets.

2013-04-15 : Note that if the spell with overload is dealing damage, protection from that spell's color will still prevent that damage.

2013-04-15 : Overload doesn't change when you can cast the spell.