My Masters supervisor always signs his emails as "Professor [Name]", but I always call him by his first name :s
It's a tough call - I always go by your supervisor first uses an exclamation mark in their emails, THEN I can use their first name! Other than that, if you know them well, and they're not stuck up, go for it. Even the most stuck up academics I've known (well, maybe except for one, but I didn't know him all that well, and he was detested by ALL) don't mind being called by their first name by someone they know reasonably well, student or not.