What open source video editing software packages are available for the Mac?
I found two:
Jahshaka
It is an open source free, video editing software, effects, and compositing suite.

When I used it several months back, it was very unintuitive and I couldn't figure out how to use it. However, there are tutorials for Jahshaka now.
Kdenlive
Kdenlive is an open source, multi track video editor released under the GNU Public Licence.

(The screenshot is of Kdenlive running on Windows, but it is available for the Mac through Macports)