Original Wording

Enbiggen: The Joker has the damage symbol.

Unpredictable Omnipotence: Before The Joker makes an attack roll, you may choose to declare a number from 1-6. If you do, and if the attack hits and the declared number comes up on either attack die, increase the damage dealt by 2 (maximum 2). If the declared number comes up on both dice, deal The Joker 2 unpreventable damage after the attack resolves.

Joker Makes the Rules: The Joker can use Outwit. When The Joker uses Outwit, he can use it normally or choose to counter all powers of a target character within 4 squares.

Errata

Unpredictable Omnipotence: Before The Joker makes an attack roll, you may choose to declare a number from 1-6. If you do, and if the attack hits and the declared number comes up on either attack die, increase the damage dealt by 2 (maximum 2). If the declared number comes up on both dice, deal The Joker 2 unpreventable unavoidable damage after the attack resolves.

Can The Joker cancel Embiggen so that opposing characters can't draw line of fire to him as a ?
No. As of the release of the 2011 rule book, powers are no longer optional and you may not choose to cancel Embiggen. (updated 6/21/11)

Does Joker Make the Rules require line of fire?
Yes. Whenever a special power uses a modified version of a standard PAC power, the only differences between the special, modified version and the PAC version are the differences explicitly stated in the special power description. Since Joker Makes the Rules does not say that line of fire is not needed, or that Joker could target a character anywhere on the map, then it will follow the normal line of fire rules for Outwit.

When I counter all of a character's powers with Joker Makes the Rules, am I countering only the powers currently showing on the dial?
No. Joker Makes the Rules counters ALL of the character's powers, including any powers that might appear at another point in the dial and are revealed via damage or healing.

If both numbers come up on the attack roll when I use Unpredictable Omnipotence, I know that Joker is damaged, but do I still deal extra damage to the target of the attack, too?
Yes. Rolling doubles only adds an additional effect (damaging The Joker); it does not say the other, previously mentioned effect (extra damage dealt to the target) is ignored.

Updated to reflect 2011 rules changes.