I am still breastfeeding my nearly 15 month old and have breastfed in public from the very beginning. I've never had any negative comments or dirty looks, infact most people (if they have realised what I'm doing) smile and politely avert there eyes. It can be done very discreetly if you use two similar coloured vest tops (lift one, drop the other), friends haven't even realised sometimes! Most places like mothercare/john lewis have rooms for nursing mothers and are very nice.
I think it works both ways really, I try and remain discreet as I understand that some people may find it uncomfortable and therefore expect the same curteousy back and for them to avert their eyes should they not like it as clearly I'm making the effort to cover up. I think the problems come when someone basically has their whole chest showing, and while I have my own opinion that it should be more acceptable in society, I understand that its not always ok with everyone and we just need to be tolerant of one another.