http://ioquake3.org/2012/08/08/ioquake3-and-os-x-mountain-lion/

The latest version of OS X, Mountain Lion has been available for a little while and we have now confirmed that ioquake3?s latest SVN (revision 2306) compiles and runs smoothly on it.
If you already have ioquake3 installed on your intel Mac running 10.6 or greater please try this build:
http://ioquake3.org/files/ioquake3-svn2306.app.zip
It should run fine under Mountain Lion, if you have Gatekeeper enabled with the default setting you will have to right click and then select Open on the .app.