I just bought a new Mac with OS X Lion and I checked in the Terminal what version of git is installed by default. I got the answer
git --version
> git ver
It would probably be better if you added:
export PATH=/usr/local/git/bin:/usr/local/sbin:$PATH
to a file named .bashrc in your home folder. This way any other software that you might install in /usr/local/git/bin
will also be found first.
For an easy way to do this just type:
echo "export PATH=/usr/local/git/bin:/usr/local/sbin:$PATH" >> ~/.bashrc
into the Terminal and it will do it for you.