Currently the New York Times has fewer print readers than Twitter followers.
"The last set of figures released, on 31st March 2010, put the New York Times’ print circulation at 951,063 and reports this week say revenue from circulation is in decline. However, with 2.6 million followers, the paper’s Twitter account is a roaring success. Particularly when you compare it to its rivals."
Following on Twitter could be something pretty passive especially for people who follow many, while physically picking up a paper (and having to go to the store to buy even more) is definitely not passive. I think a more interesting statistic to compare to print readers would be what the referring traffic is from Twitter to the New York Times site and engagement time- how long does a consumer spend reading the physical paper and how long does a vistior that comes from Twitter spend on NYTimes.com?