My youngest daughter has just turned 6, and I have been trying to get her dry at night for the last 2 years - my eldest daughter was dry at night before she was 3. I have tried most of the above suggestions, and currently I let her go to bed nappy-less but put a pull up on her when I go up (around 10.30) so she at least goes to bed like her big sister.
It has taken me this long to realise that she really has no idea she's wetting at night... it doesn't wake her at all, whereas with my eldest she woke me at 2am with a wet bed for a couple of weeks until she learnt to sleep through dry.
I think there must be something in the hormonal argument as we have tried lifting, limiting drinks and offering treats - however a dry night does seem to more luck than anything else. It's just hard when we go to friends etc and she has to put a pull up on as she's old enough to get embarrassed by it.
However, thanks to the lady who admitted to wetting the bed until she was 11, it goes to show it's something that does affect lots of people, it just doesn't get discussed in public much!