How can I use mvim to edit my crontab on Mac OS X (10.6.6)

The problem is crontab expects to be able to run a program called "/usr/local/bin/mvim -f" if you supply that in the EDITOR environment variable.

To get around that, you could write a short shell script. For example, call this one mvimf:

#!/bin/bash
/usr/local/bin/mvim -f "$@"

Then you can run: EDITOR=/usr/local/bin/mvimf crontab -e