Problem

You want to be able to use different gems/rails version for your application after installing the latest rails version, without having to use bundle exec.

Solution

You can use the named gemsets with rvm.

More instructions are here: http://beginrescueend.com/gemsets/basics/

so you can install two gemsets for example ruby-1.9.3-p0@rails3_0_11 and another with ruby-1.9.3-p0@rails3_2_1

you could then do :

rvm gemset create rails3_0_11 rails3_2_1

rvm 1.9.3-p0@rails3_0_11
gem install rails -v 3.0.11

rvm 1.9.3-p0@rails3_2_1
gem install rails -v 3.2.1

rvm gemset use rails3_0_11
bundle install

you could then also use some aliases in your ~/.bashrc file to be able to use the gemsets by using only one command as for example in ($ rvm3011):

# Rails 3.0.x
alias rvm3011="rvm gemset use rails3_0_11"
alias rvm3012="rvm gemset use rails3_0_12"

# Rails 3.2.x
alias rvm321="rvm gemset use rails3_2_1"
alias rvm322="rvm gemset use rails3_2_2"