One of the biggest offenders is scipy, Python's very good and very widely used statistical computing library. The soon-to-be-released scipy 0.9.0 does support Python 3 [0], but Python 3 was released in 2008, so it's taken quite a while.
Another big problem has been with web applications. Python has WSGI, a standard interface for implementing webapps that allows you to use a lot of different frameworks with a lot of different servers. (Similarly to Ruby's Rack and Java's servelets.) However, the WSGI standard for Python 3 (which has to be changed because the existing specification has encoding-related issues in Python 3) has only just been finalized as PEP 3333 [1].
[0] http://projects.scipy.org/scipy/roadmap#python-3 [1] http://www.python.org/dev/peps/pep-3333/
> The book went on sale in mid-August and earned out almost immediately. “Earning out” is a publishing term which means that the book has sold enough copies that my cut of the profits has paid back the advance payments that O’Reilly gave me during the writing process. Which means that I’m already receiving royalty checks for real money.
[1]: http://diveintomark.org/archives/2011/01/09/dive-into-2010
On the other hand, the text adventure game I'm writing for fun/as a learning exercise works great in Python 3, as it doesn't have any external dependencies.
Therefore, it depends if you're learning it for a specific use or just because you'd like to learn Python.
Read over http://docs.python.org/release/3.0.1/whatsnew/3.0.html for an overview on the differences. There are a few incompatible changes, but to say 2 and 3 are two different languages would be a big stretch.
All I took away from his comment was that you don't always win being an early adopter, especially when it comes to writing books. Maybe that month was an anomaly? Python 3 adoption is right where we expected it to be, and with projects like NumPy/SciPy recently adding 3.x support, the floodgates are open.
Things are going according to plan.