BackgroundDRb makes it easier to leverage DRb and takes care of a lot of details for you (assuming you're not on win32). You can use DRb directly, but you'll have to figure out ways to connect it up to your Rails controllers, daemonize a ruby process and keep tabs on currently running tasks. All of that isn't hard, it just takes time away from what you really want to accomplish.
It's still in alpha, but so far it has been solid for me. YMMV