Based on Chris Johnsen's answer:
I added this line to the [alias] section of my git config file (~/.gitconfig):
squash = "!f(){ git reset --soft HEAD~${1} && git commit --edit -m\"$(git log --format=%B --reverse HEAD..HEAD@{1})\"; };f"
Usage:
git squash N
... Which automatically squashes together the last N commits, inclusive.
Note: The resultant commit message is a combination of all the squashed commits, in order. If you are unhappy with that, you can always git commit --amend to modify it manually. (Or, edit the alias to match your tastes.)