macvim-ruby-rvm.sh

# If you already installed CommandT, remove it first and
# download a fresh copy you can build again

# check your current ruby version, if it is 1.8.7, you should be safe
# for me it wasn't :)
$ rvm list

# Output:
#     ruby-1.8.6-p420 [ x86_64 ]
#  => ruby-1.9.1-p376 [ x86_64 ]
#     ruby-1.9.2-head [ x86_64 ]

# 1.8.7 is not there, and it it the one you need, so install it
$ rvm install 1.8.7

# use it as default ! I found that just 'use' was not enough
$ rvm --default 1.8.7

# now go to your CommandT plugin and find the extconf.rb file
# should be somewhere in command-t/ruby/extconf.rb
# run it and the run the Makefile it produces
$ ruby extconf.rb
$ make

# in the output that follows, you should see that the plugin
# is built against 1.8.7 !
#
# ....gcc -I. ....rvm/rubies/ruby-1.8.7-p334/lib/ruby/... ruby-1.8.7-p334/lib/ruby/1.8/i686-darwin10.6.0 -I. -DHAVE_RUBY_H ....
#
# run macvim and type :CommandT
# all set, good to go now :)