At first my Mac was all:

~/opt/maven/bin>mvn --version
Exception in thread "main" java.lang.NoClassDefFoundError: org/codehaus/plexus/classworlds/launcher/Launcher
Caused by: java.lang.ClassNotFoundException: org.codehaus.plexus.classworlds.launcher.Launcher
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)

And then I'm like (in my ~/.bashrc)

export M3_HOME=$OPT/maven
export M2_HOME=$M3_HOME
export PATH=$M3_HOME/bin:$PATH

So now it's all:

~/opt/maven/bin>./mvn --version
Apache Maven 3.0.4 (r1232337; 2012-01-17 02:44:56-0600)
Maven home: /Users/demian/opt/maven
Java version: 1.6.0_37, vendor: Apple Inc.
Java home: /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home
Default locale: en_US, platform encoding: MacRoman
OS name: "mac os x", version: "10.8.2", arch: "x86_64", family: "mac"