I am always having trouble with installing fresh rvm instance, so here are all steps needed in fresh rvm to install new ruby 1.9.2

export ARCHFLAGS="-arch x86_64"
export CC=gcc-4.2
rvm pkg install readline
rvm pkg install iconv
rvm pkg install zlib
rvm install 1.9.2 -C --with-readline-dir=$rvm_path/usr --with-iconv-dir=$rvm_path/usr --with-zlib-dir=$rvm_path/usr