Reminds me of the conversation I had w/ my wedding photographer a while back. I asked about his digital cameras, and he told me they used 12MP cameras, and that was really the highest they needed to go. I asked if that was enough to make large, high quality prints.
He went on to say that 12MP was about right for making medium/large prints (~16x16? can't remember) without any *technical* quality loss, and for really large prints (like ones that go over a mantelpiece), those are more often viewed from a distance so the slight quality loss is unnoticeable. He used billboards as an example... from a typical viewing distance, the billboard looks sharp and clear, but if you go right up to it, you'll definitely see a difference.
He emphasized "technical" quality loss because while, in his professional opinion, the quality difference to the naked eye was unnoticeable, he's had clients that claim they can see the difference between 8MP and 12MP on 8x11 prints.