I am half-way through the Topfunky's Peepcode Code Review pdf book. It is very well written.
In my previous job, I had to find a way of processing records that was created by users and send them to an external vendor. My pointy haired boss told me to spawn a thread for this purpose. I suggested using some kind of messaging system for obvious reasons. In the end we compromised and settled for Rake task and cron job combination.
Peepcode's book talks about this issue in detail.