I found a new plugin while surfing that was created by Richard Miller. It is called What Would Seth Godin Do WordPress plugin.

Seth Godin is a marketing Guru and has written many great books. You can read our review of on of his books here.

The new plugin allows you to set  a cookie which will greet new members with a welcome message an urge them to join your RSS feed. You can set the message and number of times to show the message.

The plugin is very easy to use and I would suggest you give it a try.