Wonkette notes the "brilliant strategists" at the Bush-Cheney campaign will let you create your own custom posters for the campaign. And though the site blocks certain words like "Hitler" and "Prince of Darkness" in your campaign slogans, the prank possibilities are quite high.
Update: A Political Wire reader notes that using the "City" field, "one can type anything you like, except for punctuations and double-spaces."