I'm assuming you mean tying cron jobs to rails method calls?
Rails has a built in script called runner that you can use to call
Model methods, and there are a couple of other options as well
depending what action you want to run with the cron job.
Have a look here for a bit of a summary:
So the answer in short is it depends a bit what you want to do, for
instance I had a case where I needed to run a cron job to check the
expiry of an rss feed related model so I used script/runner
path_to_rails_app/script/runner -e production "Channel.check_expiry"
with a cron job to check this periodically.
This approach I believe is aimed at when you need to run Model methods
only, to run controller actions you may want to use something like
BackgrounDrb (http://backgroundrb.rubyforge.org/) and setup a method
to perform whatever action you need to run in the background.
Hope this helps,