Here's another summary of suggestions:

Ellenpoly – "GO AWAY!"
GUEST,Beefheart    WE SURRENDER!
Ellenpoly - PM ...You see, I was going to say "GO HOME!"
                            But WE don't want him back EITHER!
GUEST,donuel Jesus is calling you home - please go there
Dead Horse –    ---->
                         IRAQ
mooman    "This country is called Canada"
saulgoldie   Mr. Shrub, PLEASE plant yourself in Texas.
Amergin       This is Canada, the 51st State.
duellingbouzoukis    "If you take the Bible literally, why aren't you a burning Bush?"
ToulouseCruise          "Be Like Jesus -- Burn a Bush!"
Marion    Parrish in '08
(Carolyn Parrish is the government member of Parliament kicked out of the Liberal party caucus today for voicing her opposition to Bush's visit) Sorry Marion, she wasn't born in the US so can't run for presidential office)
DougR How about, "Welcome to Canada"?
WYSIWYG   You Didn't Carry Canada!
dianavan   TRY THINKING!
GUEST,amergin - How about Please Stay?
duellingbouzoukis - Canada Welcomes Careful Slavedrivers
Alba - Come for a FLU shot?
          No WMD's here either
          What do YOU want from us now?
Mooh - "FUCK OFF"
             Or how about "BUCK FUSH"?
Rapaire George, You're Not In Texas Anymore. (or Kansas-- RichM)
duellingbouzoukis - Could you borrow one from the far-right? "Why don't you fuck off back to where you came
from" Should make him feel right at home!
Ebbie    How about a simple sign citing Cheney's instruction on the Senate floor to Senator Leahy?            
Bill D the sign should say "This is CAN-a-da"...or you might get 'can-AH-dah'
Amos -   Warmongers Not Welcome!
CarolC - This country is called CUBA. You made a wrong turn in Albequerque.
GUEST,jacqui.c   Pith off?
brucie - Steak for supper, George?
dianavan -   Whats the beef?
dianavan – I BELIEVE! ...you're evil
dianavan - I can't stop thinking of these dumb signs!
LUMBERING IDIOT! Or UNFAIR TRADE or YOU KILLED DEMOCRACY!
or LIAR!   or CHEATER!
Blissfully Ignorant
Why a sign? Any of the suggestions here would just go right over his poor befuddled head. I think the best approach might be synchronised mooning...picture it; a sea of bare, upturned Canadian arses. Beautiful...and somehow poetic --(But it's COLD here in Canada now, BI!) -RichM
katlaughing - Even a dog knows what a bush is for.
Ellenpoly - JESUS WEPT
Van Lingle - FOUR MORON YEARS!
Ellenpoly - "Father forgive you...because we don't."
freda underhill   Go back- You are going the wrong way

Amos WELCOME TO CIVILIZATION
LEAVE YOUR BARBARIAN ATTITUDES AT THE DOOR
(nice one Amos--, but I'd need a banner, not a placard J--RichM )

Marion 1. When are your kids enlisting? (I really LIKE this one –RichM )
2. You know where liars go?
3. Thou shalt not kill.
4. Where's Lee Harvey when we need him?
5. I never thought I'd miss Nixon.

Blissfully Ignorant If you think we're pissed off, wait till you meet God!

brucie   Not this decade, dear, I have a headache.

dianavan - ARREST HIM! or WAR CRIMINAL! Or BLASPHEMY!

brucie - Nice suit!

Blissfully Ignorant I could lick you, bush! (yuk, too suggestive)

Blissfully Ignorant   Fee fo fi fum, I smell the blood of civilians...

Blissfully Ignorant    SHEEPSHAGGER!!!

dianavan    Domination?
                  Abomination!
mandotim   If you're the 'Leader of the Free World', how come I didn't get a vote? (I like this one too – RichM)

GUEST
How about a simple Welcome Mr Presiden, lets talk? Sarcasm is the lowest form of wit.
Ca' Nada means Nothing here, which sums it up well i think.
(Thank you , GUEST. Looks like Canada's meditations have achieved the desired state of Empty Mind --RichM)


Terry Allan Hall   "Suicide IS an Option"

LadyJean DUCK YOU FUBYA AND YOUR WHOLE FAMN DAMILY

                BUSH LIED AND THOUSANDS DIED

                BUSH IS A NEWF   
(sorry, Newfoundlanders are too cool to be used as a pejorative -- RichM)

If you can get close enough to the son of a Bush, a well blown rasberry would suffice. I don't suppose they'd let you near him with a really big supersoaker, and waterballons are right out too, ditto paint bombs.

(Son of a Bush? That's acceptable! – RichM )

Mooh
Bush Go Home, or to hell, whichever comes first.

Little Hawk-- Either home or to hell. And quickly.   
( Or , "Wishing you a speedy reincarnation" ? RichM )