Congrats on the baby--having a son is the coolest thing to happen to me.
Re your recent post on inequality--the Cox and Alm piece is a good one but the headline on it (something like "You Are What You Consumer"; probably supplied by the NYT not the authors) is unfortunate. It'll only play into the hands of folks who decry materialism, consumerism, ...
Lastly, welcome aboard!
I had another thought on the subject. Obviously there are people who believe quite strongly that disposables are more sanitary, and that they are worth the added cost: people working in medicine. Doctors and nurses use disposable gloves, clothing, even scissors, tweezers, and other tools. If it works for them, it will probably work for infants sharing sippy cups.
On the other hand, I suppose one could argue that this is an example of our health-care system encouraging inefficient, wasteful behavior, and that medical personnel should be reusing these things instead.