Thanks to everyone who commented - Richard I really like your tips for approaching speakers! As a speaker myself nothing gives me more pleasure than to know I'm making sense to someone or to pick up the thread of my talk later.
As for everyone advocating talking to people you didn't already know - this is excellent advice! I have met so many interesting people by chatting to the person next to me at a talk or in the queue for coffee! Now I almost prefer to attend events where I don't have a predefined crowd to hang out with :)