1) Download the launchd GUI, Lingon, from: http://lingon.sourceforge.net/

2) Add hourly job:

$RAILS_APP_DIR/script/runner 'ActiveRecord::Base.connection.delete("DELETE FROM sessions WHERE updated_at < now() - INTERVAL 1 HOUR")'

3) My launchd task plist file (yours will vary):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>Label</key>
        <string>us.boygeni.mysql-session-sweeper</string>
        <key>LowPriorityIO</key>
        <true/>
        <key>ProgramArguments</key>
        <array>
                <string>/users/defeated/Sites/rails_apps/boygenius/script/runner</string>
                <string>'ActiveRecord::Base.connection.delete("DELETE FROM sessions WHERE updated_at &lt; now() - INTERVAL 1 HOUR")'</string>
        </array>
        <key>RunAtLoad</key>
        <true/>
        <key>StartInterval</key>
        <integer>3600</integer>
</dict>
</plist>