Mindy - I think Mistle has described very well the positive aspects of having one child and you have said that you feel very comfortable as a mother of one. I saw something recently (was it linked to that 'have one child and save the planet' thread? I can't remember) that suggested that only children did not suffer socially and often quite the reverse, as they put more effort into friendships and social relationships.
Anyway, I can understand your uncertainty - especially if your dh seems to be edging towards the 'maybe another one' position - but I suspect that asking whether you're being fair (or not) to your dd will leave you going round in circles.
People do pop up on this board from time to time to tell us that we're being unfair to our children in not providing them with siblings. But that always seems to be coming from an assumption that siblings will be (a) close in age (b) playmates when young (c) friends and a source of solace and comfort when older and (d) sharing the load of assisting elderly parents. None of these things, though, are true - at least, not invariably. Children are often not in the least interested in a sibling who's years younger than them and there are plenty of AIBU and Relationship threads to show that sibling relationships can be blissful but are often quite dysfunctional. There are plenty of people left caring for their parents while their siblings can't be arsed pursue other priorities.
And if you turn it on its head, you could argue that having another child would be unfair on your dd, because (sorry, I forget exactly how old she is) she'd have her world turned upside down, she'd get far less attention than she's used to while you look after a tiny baby, she'll have to spend at least some of her time trailing around to baby gym or whatever when she'd rather be pursuing her own interests etc etc. That's not to say, of course, that you would be unfair on dd if you had another baby - I'm just trying to illustrate that the fair/unfair argument can be used in either direction and isn't (in my view) very helpful when it comes to deciding what to do.
What I always say on threads about this is that having a baby is too huge and irreversible a step to take unless you're quite sure that it's what you want. It's not like buying a handbag, when you can repent at leisure and send your mistake to the charity shop. Take time to think about what you really want - have you tried to picture what your family will look like in 5/10/20 years' time?
Sorry. Have droned on for far too long. Am going a little stir crazy here and starting to babble.