Well, I have a couple of stickers on my commuter - "bicycling is not a crime," and a Mammals (band) "Trad is Rad" sticker covering up the ding on the top tube. I often think of slogans but haven't figured out the logistics to make somethign visible that isn't a major wind block.
The abandoned bike on the rack here has a "Bicycling a quiet statement Against Oil Wars" plastic sign dangling from the top tube.
I've thought of a few - but there's a whole mess of 'em at http://viki.gedris.org/index.php/Slo...or_Bike_Shirts
I have also bought the paint... a year ago... to add bovinity to my now all-black Schwinn one-speed with the big baskets in the back, so that it's an utter mockery of fancy ones like the photo... with "vaca con cuernos" as the "custom" name, which my Colombian student tells me is what they call those all-purpose "milk cows with horns" where he's from.