Ubuntu 12.04 64bit LTS, running the 'rvm use 1.9.3' brings the 'RVM is not a function' warning.
Here is the case.
Multi-User RVM creates a script in /etc/profile.d, which is being sourced on startup. Also, most people put the RVM sourcing line required to load RVM in their ~/.bash_profile for Single-User installs. By default, gnome-terminal runs Bash as usual, as a non-login shell, therefore skipping /etc/profile* and executing only the user's ~/.bashrc. This means that RVM doesn't load and you get the infamous 'RVM is not a function' message.