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"