I wouldn't find any religious billboards offensive. I think having Islamic billboards is a fanastic idea, it's about time people find out what Islam is really about. As for that Archbishop, why can't Islam be displayed to the public? And those wanting to find out more about this religion can call the number or search the web, etc.
Islam is the world's fastest growing religion so therefore people are interested and want to learn more about it and as a Muslim I am glad to hear about these billboards, I hope it comes our way here in Melbourne City.