Hmmm... now it seems to be a PATH problem, indeed! :D
I've positioned my terminal at /, did what you spoke and it works, see:
fred@Fred-Ubuntu:/$ /opt/apache-maven-3.0.5/bin/mvn -version
Apache Maven 3.0.5 (r01de14724cdef164cd33c7c8c2fe155faf9602da; 2013-02-19 10:51:28-0300)
Maven home: /opt/apache-maven-3.0.5
Java version: 1.7.0_12-ea, vendor: Oracle Corporation
Java home: /usr/local/java/jdk1.7.0_12/jre
Default locale: en_US, platform encoding: UTF-8
OS name: "linux", version: "3.5.0-25-generic", arch: "i386", family: "unix"