https://github.com/mxcl/homebrew/blob/master/Library/Formula/maven.rb defaults to 3.1.0 as of now. Thus similar to https://github.com/Homebrew/homebrew-versions/blob/master/maven2.rb I saved my own formula as a gist (using mxcl/homebrew@b4725ca259a07b85e6bb20c71bcec67b7d457c42 without the detect java patch) to be able to install 3.0.x.
See also brew versions maven:
3.1.0 git checkout f160fa9 Library/Formula/maven.rb 3.0.5 git checkout b4725ca Library/Formula/maven.rb