Thanks folks (see comment thread under the question)! It turns out that I have two versions of Python under my Mac's /Library/Frameworks/Python.framework directory tree: 2.7 (came with OSX) and 7.3 (installed by EPD Free).
It turns out pip
put scikit-learn under 2.7 when I really wanted it under 7.3.
Changing directories to site-packages under the desired version of Python, and invoking pip install -t . {package-name}
suffices to install package-name under the desired version of Python.
POSTLUDE (Feb 2021): Looking back on this question from almost 9 years ago, I think I was running into a version of this XKCD comic.
Nowadays, for dependency management of Python data engineering libraries like scikit, I'm happy with just using conda and pyenv.