Seam offers some basic infrastructure for CAPTCHA creation and validation, so all you have to do if you want to add CAPTCHA validation to a form is add a single form field and show the picture with <h:graphicImage/>. The only built-in implementation we shipped with Seam 1.x and 2.0 was based on JCaptcha, but you could easily extend it and do your own question/answer thing. This is actually what I did and you can see my simplified math question CAPTCHA if you try to post a comment to this entry.
We've released Seam 2.0.0 GA to the world. (downloads and docs for the impatient) Seam 2 is a big release. We'll be talking a lot about the new features over the coming days and weeks. To get you started, I wrote up my list of the top 10 things you should know about Seam 2.