This example shows how to setup an environment running Rails 3 beta under 1.9.1 with a 'rails3' gem set.

∴ rvm update --head
# ((Open a new shell))

# If you do not already have the ruby interpreter installed, install it:
∴ rvm install 1.9.1

# Switch to 1.9.1
∴ rvm 1.9.1

# Create a gemset to contain the rails3 environment
∴ rvm gemset create rails3

# Use the ruby + gem set
∴ rvm gemset use rails3

# Download the gems(et) file:
∴ curl -L http://rvm.beginrescueend.com/gemsets/rails3.gems -o rails3.gems 

# Load the gems(et) file into the current gem environment.
∴ rvm gemset import rails3.gems

# Check to see we now have Rails 3.0.0 beta
∴ rails --version
Rails 3.0.0.beta