At some point all web applications and sites, if they’re popular, need to think about how to scale past their database bottleneck. All the big boys these days are using memcached, and with very good reason (even those not using Rails).
Rails has fantastic support for memcached and this great screencast
bad link from the guys at RailsEnvy goes over when and how you should use it in your app with some very good real life examples.
Also, if you’re still listening to FUD about Rails not scaling, this is a good place to start your ed-u-ma-ka-shun.