Marc just checked in a nifty little Rails 2.0 plugin to the OpenRain public subversion repository which encapsulates the voodoo required to use a Gmail SMTP server with an otherwise ordinary ActionMailer configuration. Gmail requires TLS security, which is why this is useful. Grab the plugin for your Gmail-mooching Rails 2.0 site, here.
Note: I previously wrote about how to do this for Rails 1.2.x here.
Update (2008.06.25): Broken download link fixed!