What's the easiest way to add a contact form for all nodes of a certain content type?
I ended using Webforms, passing the node author's e-mail using the token [current-page:node:author:mail] for passing the e-mail via a hidden field. I enabled the option to use the webform as a block and Context module for showing that block only in the desired content type.

With the lastest stable versions of Webforms and Token, I had to patch Token module as it's explained in this forum because the token wasn't working with Webforms.

http://doublemthemes.com/comment/3307#comment-3307