How about writing your message in a billboard in Piccadilly Circus, London?

Vodafone UK has a huge billboard in London, and you can use their website to send your messages to be displayed. You can send messages, establish a date and time, and arrange to meet someone in London. If you're not in a mood for a quick trip to UK, you can always browse to their webcam at the date and time (GMT) and see the message!