Cron needs to be explicitly told which RVM to use, the easiest way to do that is to pretend to open up a bash shell (as all the pre-reqs are loaded by default).
Save your afternoon and reuse the following in your crontab:
/bin/bash -l -c 'cd PATH/TO/RAILS/APP && $HOME/.rvm/gems/THE_RVM_RUBY_TO_USE/bin/rake RAILS_ENV=production THE_TAKS:TO_RUN --trace'
for me, it looks like this:
*/5 * * * * /bin/bash -l -c 'cd /srv/www/app/ && /home/garrick/.rvm/gems/ruby-1.8.7-p370@global/bin/rake RAILS_ENV=production cron:parse --trace'