Maybe something like this:
$ git diff --shortstat "@{1 month ago}"
7 files changed, 29 insertions(+), 6 deletions(-)
(As you can see, I tried this on a pretty stale repository.)
Note that this will compare the current working directory to what the current branch pointed to one month ago on your local machine.
Edit: To get stats for all commits on the branch master in a certain date range, you can use
git log --after=2011-01-01 --before=2011-01-31 --format=format: --shortstat master