I like the forum 'in-jokes' designs but thought I'd offer an alternative.
Though Matts sick x flaming cock design is amazing, and the 'life behind bars' is a great idea.
So here's something fun and friendly to promote the forum and cycling in the city. (might have to add the web address like a trail behind rear wheel). Hopefully not overly cheesy as anything with the London skyline or typical london stereotypes are generally terrible.
Nice idea! Use a black cab instead? ;)