the easiest way imho is ti use the spawn plugin.
download from: http://rubyforge.org/frs/?group_id=4646
usage is dead simple. just wrap the code to run in the background around 'spawn' like this:
spawn do
# code to run in background
end
more advanced solutions with full scale scheduling etc... might be using:
rufus-scheduler: http://rufus.rubyforge.org/rufus-scheduler/
backgroundRb: http://backgroundrb.rubyforge.org/