My post on Python IDEs got a huge number of comments, including recommendations for Eclipse with plug-ins, Visual Studio with plug-ins, and Emacs.
One of the two highest-recommended options was Komodo IDE. I've had some success with a trial of Komodo IDE on Windows, with only one major crash, which I was unable to reproduce. However, the same code brought over to run under the same version of Komodo for MacOS X produces a strange syntax error. I've filed this as bug 76114. It seems quite strange, especially given that the same code runs fine using the command-line Python under MacOS X. Despite this issue it seems overall to be a pretty usable environment.
I've also had several recommendations for WingIDE, which I will check out as well. Thanks to everyone who responded!
The program mostly reads an XML file, does a little validating and destructuring of the nodes into dictionaries, and then generates some C++ using a template. About as "real-world" a program as they come. Even as boiled-down as I can make it, it still seems awfully wordy and full of functions whose behavior is extremely dependent on my intermediate data representation. In Common Lisp, I'd be trying to use s-expressions instead of XML for the starting data, and using destructuring macros to turn the input data into data structures. With NewtonScript, I might try to find a way to do it by writing accessors that used path expressions. Is there a better way in Python? How about Haskell? I'll have to give it some thought.