You HAVE to check out Fussypants' blog entry about Nursing with Style HERE.
Basically she says that most women should avoid nursing clothes at all cost. And, to cover your tummy while you nurse with your regular clothes, buy a good quality camisole (like at the sports stores) that is thick and stretchy. Cut out holes for your "girls" and wear under your clothes.
Your tummy stays covered, and you have access where you need it (or baby does).
But, check out her blog...she has pictures and everything!