<p>What is the current state of the art in rails for preventing spam accounts? Captcha? Any good plugins, tutorials or suggestions?</p>

Spam is fair. It doesn't care what you're running behind the scenes.
So by extension, the things that work well on Rails are the same things that work for PHP, ASPNET, etc. Take a look at
Akismet
and the various "karma" anti-bot tools there are about.
For some there are existing ruby ports but you might have to rewrite a few to task.