I've been fortunate enough to visit the flight deck once during a flight from LHR to JFK last year on a 747. I had an upper deck seat and because I've seen the FD door open quite often when travelling upstairs, I thought I'd finally have enough courage to ask CC if I could visit the FD. It did help that there was a young girl visiting the FD with her mother, so I knew the crew would be ok with visitors. The captain was sipping coffee from his porcelain coffee mug and seemed to be very laid back and friendly, which made my short visit even nicer, and a lovely CC lady even took a photo of me in there. ^ It really isn't a hassle to ask to see the FD if you're flying upstairs as the cabin is small and there's not much traffic, and the crew seems to be more laid back about leaving the door open before departure (not that anyone should go there without asking the CC).

EDIT: Here's the photo!