Here's what others and I came up with. Post some of your own - perhaps it'll end up on a billboard!

"Have faith. Let go of the wheel and close your eyes - I will provide." - God
"The Devil made me do it!" - God
"I haven't been laid in 2000 years!" - God
"HINT: Darwin was right." - God
"Quit telling people what I think and know. You don't know diddly!" - God
"Do you believe? Drive off the next cliff and see if you're right." - God
"The Bible? I had nothing to do with that obscenity" - God.
"I'm as real as the Easter Bunny." - God
"I let your Mom die a slow, horrible death." - God
"Because I said so, that's why!" - God
"I've spoken to Jimmy Swaggart and Oral Roberts, but I won't answer your prayers." - God
"I let my son be crucified. Why do you think I care about you?" - God
"If I really wanted to talk to you, why would I resort to a billboard?" - God
"Stop bugging me." - God
"Sleep in on Sunday mornings." - God
"Note to all you athletes: I don't watch sports." - God
"Kill fags." - God
"I knew Adam and Eve would succumb and doom the human race, but I tempted them anyway." - God
"You aren't responsible for your actions, Jesus is." - God
"Worship me, or I'll torture you forever." - God
"Do what I say or I'm taking my universe and going home." - God
"If you love someone, threaten to burn them in hell for eternity." - God
"An apple a day leads to death and suffering." - God
"Don't blame me. My budget was cut." - God
"...and did I mention that I've recently become a Buddhist?" - God
"Pay no attention to that man behind the curtain." - God
"Hitler? Stalin? Pol Pot? Pah. Amateurs." - God
"Sorry. I'm not speaking to you." - God
"Hi everyone, I'm back! So, did the kid behave himself?" - God
First billboard: "Q: What do Dennis Rodman and I have in common?" - God
Second billboard: "A: We both have slept with a Madonna." - God
"So who's this God guy anyway?" - Zeus
"Free - lifetime supply of SPF 666 suntan lotion." - Satan
"DON'T do as I DO...do as I SAY" - God
"We apologize for the inconvenience." - God
"That part about "Women, be submissive"? I meant that." -God
"Don't ask "What would Jesus do". Ask, "who would God kill?" - God
"You think YOU'RE a victim of child abuse?" - Jesus
"Nietzsche was right." - God
"Thou shall have no other gods before me." - Allah
"I could prove my existence, but then nobody would get to go to hell." - God
"That part about rabbits chewing cud - I meant that." - God
"Men!" - God's wife
"Hey! I just realized that GOD is DOG spelled backwards. Where is that Adam?
"I'm going to tear him a new asshole...again." - God
"Bats are to birds, don't mess with my inerrant word." - God
"I've been pretty much the same since I dropped that house on your sister."-God
"Having sex? I'm watching." - God
"Columbine? I was at lunch." - God
"Does your life suck? It's all part of the Plan." - God
"This is just a test. If this had been a real life you would have been notified on how to live." - God
"Remember that 'Love Thy Neighbor' thing?... I meant it... IN THE PLATONIC SENSE!" - God
"With Me AND the dog watching, I'm surprised you two can do it at all..." - God
"Six Billion?!!!! Jesus Christ what the hell was I thinking?" - God
"Of course you don't question my existence. I'm not invisible." - Man
"Hell is cruel and unusual punishment." - Man
"Noah's ark? You've got to be kidding!" - Man
"Love means forgiving even those who don't worship you." - Man