On Linux it's Monospace 10 pt. (the exact monospace font used may vary on different Linux distributions or versions), on Windows it's Consolas 10 pt., and on OS X it's Menlo Regular 12 pt.
(The color scheme is Neon, the syntax highlighting is from PackageDev, and the font is Liberation Mono
This information is found in the Packages/Default directory (where Packages is the directory opened by the Preferences ? Browse Packages... menu option), in the Preferences (OS).sublime-settings file where OS is one of Windows, Linux, or OSX.
You should only customize the font (or any other setting) in Packages/User/Preferences.sublime-settings, opened by Preferences ? Settings—User, as Settings—Default is over-written on upgrade, and also serves as a backup in case you really screw something up in your user settings. This is the case for both the main Sublime settings as well as those for extra packages/plugins.
These default fonts are the same in Sublime Text 2, Sublime Text 3, and the new version currently in development.