Is X11 installed by default on Mac OS X?

I'm asking this because I'm considering distributing a commercial Windows application to mac users. The application would run in WINE, but WINE depends on X11.

I've read that X11 is installed by default on OS X 10.5 and later, and available as an optional install on the installation dvd for OS X 10.4

Can I reasonably expect X11 to be available on most of the users' computers? If not, how difficult it would be to install for the user? Can I just provide them a link to download and install it from apple's website?

YES it's installed by default. It's in the installation options where users can tick or untick to install X11. But by default it's ticked in 10.5, 10.6 and 10.7

The previous answer seems to be referring to Xcode, rather than X11. While Xcode is now downloadable from the App store, X11 is installed automatically when you install Lion.

If any of your users have problems, or find it not installed on their system, they can download the installer here:

http://xquartz.macosforge.org/trac/wiki/X112.6.3

As from OS X 10.8 X11 is not installed by default.

However there is an easier way to install. Use Wineskin to wrap your app. Then you will get an OS X app that includes X11 and your app