Is there an easy/efficient way to generate some high-level reports for a particular GitHub project?
e.g. lines of code committed over time, top contributors, etc
You can try applying gitstats to a cloned repo of your GitHub repo.
See for example the activity of the Git repo itself, with many data like the commits per year/month:
See also "Looking at Git Statistics", with the GitHub 'yearly_stats' for a simple little script.
Update April 2013 ("Get up to speed with Pulse"):
Check out Pulse
Pulse is a great way to discover recent activity on projects.
Maybe you haven't checked on your favorite repository in a while or you just discovered a project that looks interesting to you. Pulse will show you who has been actively committing and what has changed in a project's default branch:
You can find the link to the left of the nav bar.