magit-present-log-line uses window-body-width that doesn't exist in Emacs 23.3. If there are commits that would be displayed in magit-status-mode, this causes an error when magit-status command is used to start magit. Emacs 23.3 is what the latest Ubuntu LTS (12.04) version provides, for example, so I think it is important to support that.
I hit the same problem on Aquamacs - here's a simple workaround I added to my init file:
(unless (fboundp 'window-body-width)
(defalias 'window-body-width 'window-width))
I'm not 100% convinced it does exactly the right thing, but it seems to work for me.
should be fixed in 14b7350