People have mixed views regarding covering up while breastfeeding. For me, I cover up when in public places where I may not feel completely comfortable, but I am slowly learning to feel comfortable in more places, so I don't need to cover as often. I have known people who felt 'offended' by women just whipping out a boob and feeding their kids in public. While I have learned over time that it isn't offensive (I used to be not nearly as open-minded), I wouldn't want someone else to develop a negative opinion of breastfeeding for that reason alone.
I'm actually trying to create my own cover with a more breathable fabric, some cinching on the bottom, and a neck thing that won't strangle you when tightened. In the mean time, I found this information on someone else's blog:
Free Udder Covers (just pay $10 shipping). They normally cost $32.00 but you can use the coupon code Family2010 at checkout and the amount will be automatically deducted. The coupon will work on any item in their store.